James D. G. Dunn's Unity and Diversity was a blockbuster for me. I devoured it when it came out in the late 70s. It was both an advanced introduction to the New Testament (not authors and dates, but concepts and origins) and an exploration into the validity of speaking in terms of orthodoxy and heresy in earliest Christianity. Listen to these words from the opening paragraph:
The relation between orthodoxy and heresy has always been important in the history of Christianity. Orthodoxy has traditionally been thought of as conformity to 'the apostolic faith.' Up until the twentieth century the tendency has always been for each church, denomination or sect to claim a monopoly of this faith, to deny it to others, to ignore, denounce or persecute the others as heretics.
At some point, I wrote in the margin "the c of c approach," a painful reminder of my Church of Christ experience.
Dunn goes on to explore the unifying elements in the New Testament. He concludes that THE unifying element was the unity between the historical Jesus and the exalted Christ. And he concludes that there was no single normative form of Christianity in the first century. Various Christianities are explored--Jewish, Hellenistic, Apocalyptic, and Early Catholicism.
If you're ready to move beyond the standard introductory text book fare, here's a book that you can learn from each time you pick it up. This is not a radical repudiation of Christianity, but a substantial historical/theological look at how a new religion developed.