The study of heresies is an interesting one, partly because the ideas that were ruled out of orthodoxy tell us a lot about the nature of orthodoxy, and partly because, particularly in the case of the early heretics, they were at least partly responsible for the very formation of orthodoxy: often they put forward ideas that, at the time, seemed (at least to them) congruent with the teachings of the Church, and so forced a discussion about whether or not what they said could be considered Christian, and if not, why not.
One of those early and influential heretics was Marcion, who led a briefly prosperous sect which was excommunicated in about 144 AD. We know about him mostly from Christian apologists like Justin, Tertullian and Irenaeus, who wrote treatises about why he was wrong. At the heart of Marcion's thought seems to be a struggle to reconcile the Old and New Testament. Marcion decided that it was impossible, and argued that the Gods of the Old and New Testament were in fact two Gods, rather than the same God. The God of the Old Testament was a Bad God, "the author of evils, a lover of war, inconstant in judgement, and contrary to himself", who created the world. The Father of Jesus was a superior God who offered salvation through the escaping of the physical world, and human bodies which were unredeemable. Marcion was, as a result, the first to suggest an official Christian scriptural canon, which he thought should consist of the gospel of Luke and some of Paul's letters, both heavily edited to get rid of any passages which suggested continuity between the Father of Jesus and the Creator God of the Old Testament.
I think Marcion's interesting for a few reasons. Firstly, because he does the thing that heretics often do, and goes for the easy theological way out. The Old Testament is difficult, presenting a God who, at times, can seem like a bit of a git. What do we do? Heretical andswer: get rid of it, and keep the bits we like instead. A theme of the Church's slow formation of doctrine seems to be that they go for the difficult answers, the messy solutions that often feel less like solutions and more like the permanent inscription of difficulty into Christian theology. Partly as a result of Marcion, the slow emergence of a canon of Scripture begins around this time, saddling us once and for all with a Bible riddled with difficult questions, apparent contradictions, and a story which can feel pretty alien to the God we think we know. Loads more fun, of course, and probably a lot more useful in helping to engage with a world that's equally tricksy and befuddling, but you can't help sympathising a bit with old Marcion, not least because I think that we're almost all guilty of doing what he did and ignoring the difficult bits of the Bible.
Also, I think that we see in Marcion another regular theme of early Christian thought: the attempt to reconcile the teachings of Jesus, the Hebrew Scriptures and the apostles with contemporary Greek thought. Marcion's idea of two Gods bears some resemblance to the thought of Numenios, a Middle Platonist, although on reflection I think we'll come back to that some other day, as that's a whole big barrel o'worms. But for now, suffice it to say: there are lots of arguments now about the Greek philosophical legacy in Christian thought - how influential it is, whether its influence is a good thing or a bad thing - but I think the case of Marcion shows that, at least some of the time, Greek philosophical ideas were rejected, however appealing, because they simply couldn't be made to fit into Christian orthodoxy.