Today's scholars see amazing diversity in the first few centuries of Christianity. It lasted until the Christians obtained temporal power with the conversion of Constantine. I just finished reading through the short book The Beliefnet Guide to Gnosticism and Other Vanished Christianities (2006), which summarizes some of those early "heresies," starting with the gnosticism. Here's a short tour based on the book.
Gnosticism was the first real movement. It comes in two flavors, Sethian gnosticism, the "pure" gnosticism, and Valentianian gnosticism, the Christian variety. But it's a mistake to see clearcut divisions or groupings as there was little institutional organization among the gnostics: it was all fairly eclectic and all gnostics shared a few common themes. As noted, gnosticism wasn't all just a Christian offshoot. Some of the gnostic writings were enlightening and uplifting and insightful, but the whole way of thinking appears hopelessly mystical to a modern reader who's not also a New Age type. What's amazing is that Valentinus, a Christian teacher who lived and taught at Rome for a number of years, was actually in the running to be elected Pope around 160 AD. That gives you an idea how mainstream gnosticism was before there was an accepted Orthodoxy. They had their own spiritualized version of Christian theology but, in organizational matters, just piggybacked on the mainstream Christian church.
The Marcionites shared some of the doctrinal ideas of gnosticism, but they were fundamentalists and simplifiers, not elitists like the gnostics. They simplified the canon by throwing out the gospels except for Luke and kept only Paul's letters, then edited even these writings. Marcion threw out the Old Testament entirely. His response to differences between the NT books and the OT was not to develop a sophisticated exegesis or symbolic interpretations, but simply to throw out what he didn't like and rewrite what he kept. But Marcion's truncated canon did spur mainstream Christian thinking (by Iranaeus) about what the canon should or shouldn't include.
The Montanists were like modern Pentacostals, full of the Spirit and basking in ecstatic experience. Women occupied very visible and active roles in Montanist congregations. It was not so much their doctrine as their quest for rigor and spiritual zeal that put them at odds with Orthodox leaders. They were not intellectual elitists like the gnostics, they were purists who thought the mainstream church was getting lax and decadent. The famous Christian apologist Tertullian, something of a rigorist himself (most famously in his phrase "Creo quia absurdum"), ended up joining the Montanists.
The Donatists sprang out of a specific historical episode, the Diocletian persecution from 303 to 305 AD. When it was over, the mainstream church was willing to receive penitent traditores — individuals who had, under the pressure and threat of the persecution, denied the faith — back into full fellowship, but the Donatists, hardliners, would make no such compromise. Not only were they ethically demanding, they were willing to fight for their program and did not shrink from martyrdom (which was something of an ideal for them, given their roots in the Diocletian persecution). Thus roving bands of armed peasant Donatists would attack Catholic landowners, clergy, and (of course) tax collecters. The eventual eclipse of the Donatists and the victory of the Catholics (which means "universal," after all) was a victory for universal church membership over the narrower vision of a Donatist "church of the saved." They sound something like armed and angry New England Puritans running around beating up colonial Anglicans.
Outside the border of Christian thinking and practice were the Manicheans (cosmic dualists founded by Mani, a Persian) and the Hermeticists. The Manicheans wrote beautiful devotional literature and inspired followers to serve the Good in the struggle between good and evil. No less a thinker than Augustine studied then joined the Manicheans for several years. Hermeticists idolized Hermes Trimegistus, the Greek name for Thoth (the Egyptian god of knowlege, which extended to science, religion, philosophy, and magic). These elitists trained to acquire and teach esoteric knowledge and scriptural insight. Rediscovery of lost manuscripts during the Renaissance reintroduced Hermetic thinking to the West and it has been with us ever since. It's the sort of mystical, alchemical gobbledygook that some people really fall for, but what do you expect in a world where every newspaper except the Daily Universe features daily horoscope reports (because readers demand it) and millions of people think they'll actually win the lottery sooner or later if they just keep buying tickets?
Then there are the Neoplatonists. They carried the torch for what was left of philosophy in the waning years of the Roman Empire. Personally, I find the Neoplatonists to be as mystical and ungrounded as the Hermeticists, but those who know better insist there is real philosophy at the core of Neoplatonism. And it didn't just whither and die along with the rest of the ancient world, it leaped cultural hosts to Christianity: "Christianity became in the end a theological child of Neoplatonism — most of the early theologians who framed the doctrinal basis for orthodox Christianity, especially regarding the incarnation, the Trinity, and the contemplative life, were influenced by this philosophy."
Neoplatonism left its imprint on the Nicene Creed, too (see my post from earlier this week). I'll close with the final lines in the book, which stress the extent to which Neoplatonism helped define Christian orthodoxy.
The word used by the orthodox bishops to describe Jesus' status vis-a-vis the Father was homoousios, "being of the same essence," as opposed to homoiousious, "being of similar essence." The distinction may be a subtle one, but to the Neoplatonically trained bishops it was immensely important. The first, "being of the same essence," identified the Son with the primordial divine essence that created all beings. The second, "being of similar essence," created a kind of gulf between the primordial divine essence and the Son, leaving room for the notion that the Son was a lesser being. It is virtually impossible to understand the Nicene Creed without first understanding the philosophy of late antiquity. They are completely fused and interrelated.