Over at Canon Fodder (which is the best name I’ve heard for a blog), Michael Kruger has been discussing misconceptions of the NT Canon. In his latest post he discusses the basis for distinguising heresy from orthodoxy in the early church. I really enjoyed what he writes concerning the role of the Old Testament in the early church.
Routinely overlooked by those in the Bauer camp—ironically in a Marcionite fashion—is the decisive role played by the Old Testament amongst the earliest Christians. M.F. Wiles once declared, “There was never a time when the Church was without written Scriptures. From the beginning she had the Old Testament and it was for her the oracles of God.” Aside from the numerous examples of Old Testament usage within the New Testament itself, quotations from the Old Testament are abundant within the writings of the apostolic fathers and other early Christian texts. Thus, right from the outset, certain “versions” of Christianity would have been ruled as out of bounds. For example, any quasi-Gnostic version of the faith which suggested the God of the Old Testament was not the true God but a “demiurge”—as in the case of the heretic Marcion—would have been deemed unorthodox on the basis of these Old Testament canonical books alone. As Ben Witherington has observed, “Gnosticism was a non-starter from the outset because it rejected the very book the earliest Christians recognized as authoritative—the Old Testament.” So, the claim that early Christians had no Scripture on which to base their declarations that some group was heretical and another orthodox is simply mistaken. The Old Testament books would have provided that initial doctrinal foundation.
 M.F. Wiles, “Origen as Biblical Scholar,” in The Cambridge History of the Bible: From the Beginnings to Jerome (ed. P.R. Ackroyd and C.F. Evans; Cambridge: Cambridge University Press, 1993), 454.