Let's take a look at the movements and ideas that the early Christian churches came to regard as 'heresies'. These are mostly gleaned from historical sources that mention them.
(There is a different page on what is meant by
'heresy', and arguments about the value of the concept of heresy. I use 'heresy' here with the intent described there, despite its problems, also described there.)
Arianism is the teaching that Jesus was not
eternal, that he was a created being, created within the framework of earthly space and time, and thus not fully God.
Arius was a priest in Alexandria, Egypt, who died in AD 336. Mind you, Arius thought that he thought highly of Jesus: Jesus was seen as the very best creature that God created. (This was not an entirely new idea; the Ebionites and Gnostics had gone in a similar direction.) His teaching caused a stir in his day, and in some Asian cities, most church members held the beliefs of Arianism. Arius had triggered such turmoil within the churches that it was a leading cause of the Council of Nicaea in 325 AD. The Council examined the Scriptures that spoke of who and what Jesus was, in Paul's letters and the first chapter of the Gospel of John, and declared Arianism to be a heresy. They wrote a creed that was mainly aimed at opposing Arianism. The struggle continued after Nicaea and after Arius' death, and it even took on political implications. The chief of the opposition to Arianism was Athanasius. In 381, the Council of Constantinople once again called Arianism a heresy, mostly due to the biblical and philosophical case that Athanasius had built over the years. Arianism faded fast after that.
The specific Arianist group Athanasius fought had died, but the basic idea did not. It resurrected itself many times throughout church history. Many of today's groups that derived from Christianity have strong tendencies toward that belief, even if expressed very differently: Unitarianism and Jehovah's Witnesses, among others. In mainline US and European Protestantism, it feeds into the universalized-Protestant openness toward other beliefs ("Christianity is one of many paths to God"). It has much appeal to those who base their beliefs on highly rational approaches. The question is, rational or not, is it true in any way? For if it is, you might as well be Unitarian, or maybe even Jewish. Christianity is, before it is anything else, trusting that Jesus of Nazareth did what no other could do, what we have time and again shown we are unable to do for ourselves. By His life, death, and resurrection, our wrongs are forgiven, and our relationship with our Creator is being restored and healed. Arianisms, ancient and modern, say 'not so'.
Docetism [ < Greek dokeo (to seem, appear to be)] : The idea that Jesus Christ
was a totally divine being who only appeared to be human. It is the opposite idea to Arianism, in which Jesus was all-human and not in any way divine in nature. For docetists, Jesus was with us, but never really of us, and thus didn't really suffer, and (for most docetists) didn't really die.
Docetism is not a heretical group as much as it is a category, a basic idea which describes many schools of thought over the years. (See "Apollinarism" below.) Any idea that assigns a low worth to the material world and a high value to that which is deemed 'spiritual' will tend toward this idea. Docetism is a form of dualism, and like other dualisms, Docetism was rejected by the early church as not being in keeping with what Christ was about. God so loved us that God 'incarnated' (that is, became human) in Jesus, thus blessing the whole 'material' realm with the 'spiritual' impact of God. But like most heresies, Docetism tries to come in through the back door; the churches, especially parts of Roman Catholicism, and fundamentalist Calvinists often draw a vigorous sacred/secular divide.
Ebionitism [ < Heb. 'ebyônim (the poor) < 'ebyôn (a poor person)]: The idea that Jesus was a human being and not at all divine, but Jesus was given certain gifts by God's spirit which set him apart from other people. Because of what God gave Him, Jesus was deemed the Messiah or Christ, but that meant only that He was chosen by God, not that He was savior of all humankind. The Law of Moses was still what counts with God. The church rejected the idea behind the Ebionites, possibly as early as 110 AD.
Some scholars have made the claim that the Ebionites were pretty much the lineal descendents of the Jewish Christians that were led by James the Just at the time of Paul. (These scholars posit a more adversarial relationship between James himself and Paul than Acts or Paul's letters show.) However, it would be much more accurate to put them in the context of other groups, both Jewish and Christian, that existed during the first hundred years of Christianity. There were many small Jewish groups (like the Essenes of Dead Sea Scrolls fame) with many different shades of practice, with many ideas and practices which bled into those of other groups, making a continuum. Like Judaism, Christianity also had small groups which took a different direction than the mainstream. Nearly all of these were moral rigorists in some way -- their behavior had to tightly conform to the Law of Moses, seen to varying degrees through the lens of Jesus' sermons. And only some of them were rejected by mainstream Christianity.
The Ebionites were larger than most such groups, though not reaching much more than 1000 or so adult members as far as we can tell, and they had different shades of practice amongst themselves. They lived in small communes, where they had no personal material possessions. The gospel they used appears to be Matthew's, but evidence for it is scanty. To the Ebionites, Jesus was the messianic prophet or archangel, adopted by God, not God Himself. Jesus did not exist before creation, his death did not free humanity from the price for sin, and he did not return from the grave. They self-identified with "the poor" that Jesus spoke of in the gospel of Matthew, hence their name. They rejected animal sacrifice, as well as the Jewish oral laws that were developing at this time (which led to the Mishnah), and anything else outside of Jesus that added anything to the Law. The Ebionites were not the only ones who held these beliefs. Similar groups continued to exist in the Middle East until at least 800 AD, if Muslim sources are correct, and the Mandaens of modern years are originally from that milieu. There is good reason to believe that Mohammed had contact with their ideas when he lived near Palestine. No group of today is descended from the Ebionites, though some "messianic Jewish" groups bear some resemblance to them. In some key ways, the Ebionites, for their time, were almost as Jewish as they were Christian.
Apollinarism or Apollinarianism : Since the Nicean Council in 325, Christian theological-talk focused on what the Trinity meant, especially to explain how Jesus Christ could be fully God and fully human at the same time. Apollinaris the Younger (d. 390 AD) became bishop of Laodicea (in Syria) around 361 AD. He championed the idea that Jesus' mind was solely divine and not human, that the creative Logos of God had in some way taken the place of the human Jesus' mind. His human nature was confined to His body (physical nature), but even that was in a sense 'glorified'. This made Jesus almost entirely divine and not fully human.
The trouble was that the churches had come to see in Scripture Jesus' role in relating not just to God within the Trinity but to humans as a human. He was on both sides of the Divine/Human relationship. It was the only way he could effectively reunite and reconcile the two sides. Or, to use the language of the sacrifice: he was fully God, in order to be a pure and worthy sacrifice, and he was fully human like us, so he could be the sacrifice for us. Apollinaris' approach makes Jesus un-like us and not quite like God, a third kind of being. Athanasius showed how Apollinaris' approach poorly explained what Jesus Christ did, at a Synod (gathering of regional leaders) in Alexandria in 362. The "fully God, fully human" approach of Athanasius won the day in 381 at the Council of Constantinople. The church, almost as a whole, accepted what the Council determined on the matter, leaving Apollinarism to a small set of followers, which included the Polemians. And it wasn't long before even those faded away.
Eutychianism: In the Eutychian way of thinking, Christ was put through a blender - that is, His being God and His being human were so totally mixed together that He was homogenized into being Something Else. When theologians say that Christ has two natures, they mean that Jesus was fully God (and thus had the full power and ability to accomplish His mission of rescue) - and fully a human, living out a material human life (which made it possible to experience life such as we do, thus to be in full solidarity with us, and to redeem us out of love). Jesus is one of us, and that is what makes what He did so real, what makes it matter. No third type of being could do it for real, because it wouldn't be on either side of the relationship that was to be healed.
gnosis [ < Greek gnosis (knowledge)]. The Greek word gnosis is a basic word for any sort of knowledge. In a religious or philosophical context, it usually refers to the 'secret' or 'special' knowledge that is said to set one free from the 'illusory' material world. There is no such knowledge, according to Christian beliefs:
the material world is not an illusion for us to be freed from, it is
a wonder-work of God's creation we were made to be a part of, and God is still at work today to make it whole again;
the knowledge on which everything hangs is already revealed by (and as) Jesus Christ, and God sent the Spirit to
reveal to all believers what God is up to -- this is no 'secret', and even if it was, the Christian's duty would be to give it away freely to everyone anyway.
gnosticism: Gnosticism arrived when Christianity was a mere toddler. It was the offspring of the mystery cults and religions of Greece, Persia, and Palestine, with Jewish sectarian apocalypse and perhaps even stories from lands further east of the Roman world. It tried its best to suck in the strong, fledgling Christian faith and reshape it into its image.
Keep in mind that there were many different Gnostic groups, so there were many different flavors, mostly alike but not quite the same. However, there were certain core markers. The descriptions found here is an 'average', what is most typical of them.
In gnosticism, the spiritual world was full of deep secrets. Matter was evil or unreal or at best icky, and spirit was good. Terms like 'vessel', 'container', 'jacket', 'package', 'can', or 'vehicle' are how they would today describe the (dispensible) human body. Some spirits were more 'good' than others, and part of the task (dare I say 'game'?) was to keep these spiritual superiors happy. A few people (that is, themselves) were learning about and evolving toward the spiritual world. A follower's task was to stay focused on spiritual things and to ignore or not value those who lacked such 'knowledge' or 'consciousness'. That knowledge had nothing much to do with relationships (except with those who 'know' more), or emotions. The meaning of holy writings (including some of their leaders and pagan writers but not including most of the Old Testament) was uncovered by way of allegory ("this really means *that*"). The gnostic world-view was sharply unlike that of the Hebrew Scriptures.
The Gnostic "gospels" include books such as the Gospel of Thomas, Gospel of Mary, Gospel of Truth, and Dialogue of the Savior, and were written in the late second century AD. Most of these writings were found at Nag Hammadi, which was a religious library of sorts, not an attempt to collect a new set of Gnostic scriptures (as some popular writers would have it).
For the most part, these 'gnostic gospels' are not stories told about Jesus, or reports about his life. They're not about him giving grace, or being up close and personal with people. The Gnostics' writings deal with Jesus' death hardly at all. The Jesus portrayed by the gnostics gives out spiritual sayings or responds to questions, mostly left alone without a context. Or, he is dealing with the realm beyond the material world, or those in it. There's not much of a setting - it's as if no life was lived behind the sayings. The Gnostic Jesus has little about him that marks him as a Jew save for some apocalyptic symbolism. This could not be more unlike the Synoptic Gospels, which show Jesus at synagogue and at the Temple and in prayer, turning to the Hebrew holy books, as one would expect from the Jew he was. God as a human?? For a gnostic, God would never stoop so low as to be contaminated with our filth. Thus, of course, they had to re-translate Jesus: he was a human who had developed the highest level of contacts in the spirit world. His body either became, or always was, an illusion. The gnostic "Jesus" is one who saves you from the world, not for it. A Gnostic god could never really be 'God-with-us', nor could it be so focused on loving those wallowing in the earthly mire.
Old-fashioned Gnosticism morphed into new-fangled New Thought (Unity, Christian Science) and then New Age (Celestine Prophecy, and especially Urantia), and left its traces in other places (in some of the Word of Faith preachers, in A Course on Miracles, in the late Heaven's Gate, among some religion scholars, and in much of US pop religious culture). For reasons that are baffling given its lack of logical sense, the idea of 'secret' or 'special' spiritual knowledge has historically had its strongest appeal among the intelligentsia and the social elite. The best explanation is that they badly want to believe they're a step above others. Modern gnostics know how to dress up their ideas with a 'positive' spin about our selves and the earth. But it's still spin, and still winds up draining into the same hole.
See what secret knowledge the dictionary has about gnosticism. And how certain modern gnostics (who are not descendents of ancient ones) explain themselves.
Pelagianism : The belief that human beings can, by their own effort, keep themselves from sinning and thus be able to (at least in some sense) save themselves. It's named after a fifty-century monk and teacher whom Augustine of Hippo accused of teaching this belief. Most churches have had a streak of Pelagianism running through their theologies at some point in time, as they grapple with the issue of how capable each human is to make choices and determine the course of their own lives. Pelagius himself may have been slightly less than Pelagian, in that he appears to have taught that God gave the baptized believer the ability to follow His commands in full, rather than all humans being able to completely do it totally on our own. Either way, Pelagianism leaves no allowance for us to sin; we could live sinlessly, and only those who did so belonged in God's presence. The main thrust of Augustine (and the apostle Paul) was that we all live as sinners who even at our very best fall well short. We can't and won't do it, and never even fully want to do it. All human beings depend on Jesus to pay the bills for our mess. And we can't even accept this gift from God without God's Spirit at work within us to tell us it is there for us. Augustine saw the Church as a
hospital for sinners and a school for following Christ, all by way of God's grace.
A belief that usually goes with Pelagianism is the idea that we are all born innocent, with no sin at work in us from the start. Mainstream Christianity holds that sin is a part of our nature, and thus we all start with it. Some Christians (including most Baptists) treat young children as being 'innocent', not because they are without evil but because they are not yet responsible for what is wrong in them.
The New Testament is not big on naming heretical groups. Instead, it usually describes why a specific belief or attitude is not in keeping with what Christ taught. However, in Revelation chapter 2, a specific group is named twice - in the messages to Ephesus and to Pergamum. The group, the Nicolaitans, is named, and it is said that the Lord hates their deeds (Ephesus, Rev 2:6), and they have teachings which the Lord is at war with (Pergamum, Rev 2:14-16). The way the latter is phrased ties the Nicolaitans to the practices taught by others who are given the symbolic name of 'Balaam', which is a way of describing by pointing toward a Biblical figure who did things which had the same thrust behind it. (Balaam helped lead Israel into idolatry committed through cultic sexual acts, and through these acts, into a plague.) The Nicolaitans were not the Balaam group, but were having a similar effect on believers in Pergamum, which infers their beliefs were much alike. The Nicolaitans must not have lasted long (Eusebius agrees), nor grown beyond their home base in what is now mid-west Turkey. Little else is known about the Nicolaitans or their heresies. Irenaeus adds only that they lived lives of "unrestrained indulgence"; most other reports copy Irenaeus. Hippolytus of Rome says that it was founded by and named after the deacon Nichola(o)s, by which he probably meant the one named in Acts 6:5. Clement of Alexandria, however, disagrees, writing that deacon Nicholas' words were twisted in order to make them seem like he supported such practices. There is the possibility (which may lie behind Clement's report) that there is no connection between the Nicolaitans found in Revelation and the somewhat Gnostic group these writers and Tertullian knew about. In all cases, the group(s) called 'Nicolaitans' had pretty much passed from the scene already by the time the writers wrote, hence their confused reports.
Despite the lack of direct knowledge about the Nicolaitans, there's no lack of theories. Some end-times theorists lump them together with the 'Balaamists' as well as 'Jezebel' (from Revelation's message to Thyatira, 2:20), making them a sign of what is to grow from within the church in the last days. Against that: unlike Balaam and Jezebel, there is no Nicholas or Nicolaos of any kind in the Hebrew Scriptures, and no such figure of evil in the rest of the New Testament.
Do not confuse the Ebionites with Ebonite™, a brand of bowling ball. Nor Docetism with Deux-ce-deux-tism, a belief in the magical powers of square dancing. Nor Pelagianism with Plagiarism, using other's creative works and claiming to be its author. Nor Montanism with Montanaism, fanship for a former '49ers quarterback. Nor Nicolaitanism with the worship of Santa Claus (St. Nicholas). Gnosis has nothing to do with your (g)nose. Arianism has nothing at all to do with Aryanism, the idiotic racist belief in the superiority of the Indo-European peoples.