This is further reflection on poly- or pluro- doxy given the comment by José in an earlier post that we don't need new language - that the old is good enough.
No it is not. Why? Because there wasn't an orthodox Christianity in the second or third centuries from which others deviated and were heretics. This language only works if (1) you have an established historical orthodoxy that dominates the scene or (2) you use it in terms of a theological self-reference, as in my way is orthodox and yours isn't.
Now some of my readers might like the apostolic church and identify with it, and therefore say that there was an orthodoxy and the apostolic church was it. Everyone else is a heretic. Fine, but this is not a historical perspective. It represents the reality of #(2), not #(1).
For instance, let's take Marcion. From a historical perspective (not the apostolic Christian one) Marcion in his era was as much a Christian as anyone else. He established a very viable church with the first NT canon! Jesus Christ was the redeemer. His theology was a radical exegesis of Paul. Now Tertullian hated him and so did Justin Martyr and Irenaeus, all who identified with the apostolic church and all who saw him as heretical. But thousands upon thousands of people loved him, thought him a brilliant Christian theologian, and were members of his churches. These same people saw the apostolic church and its theologians as ignorant. In many parts of Asia Minor, Marcionite churches were not just the mainstay, but the first churches established. They were productive into the 10th century according to Arabic reports.
Let's take the Gospel of Judas. Here is a gospel written by someone in the middle of the second century who knew for certain that the apostolic church was run by dupes. This person considered his own views about Jesus and salvation, and his own practices (some type of water ritual), to be the only way to God. He considered himself a gnostic Christian and was quite offended that the apostolic church would be using Jesus' name in such a disgraceful (and demonic) manner - to offer a sacrifice to the lesser god!
As historians we cannot be theologians. The texts tell us the story. And this story was a story of many competing orthodoxies, all who claimed for themselves the "Christian" name. At least in the pre-Constantinian period, the marking of a heretic comes from within each of these orthodoxies, and represents their individual understanding of what it means to be the "real" Christians.