There were Valentinians, Marcionites, Ophites, Cainites, Docetists and also a church that followed Jesus but held to all the Jewish traditions.

The Bible that has been accepted as canon is the result of the triumph of one particular interpretation of who Christ was and what his message meant. It doesn't take long to figure out who that person is, since the bulk of the New Testament was supposedly written by him - Paul.

There are plenty of fascinating books that describe the controversies of early Christianity. Lost Christianities by Bart D. Ehrman is a good starting point, though I don't really like some of his terminology, which implies that the existing canon is the "right" choice even though his book doesn't. Elaine Pagels' The Gnostic Gospels talks about the nature of the alternative systems, and Who Wrote the New Testament? by Burton L. Mack is a very well-written explanation of the development of the ideas that came to dominate Christianity. Some more academic works include C.W. Griggs, Early Egyptian Christianity, A. Yarbro Collins, Cosmology and Eschatology in Jewish and Christian Apocalypticism, Alan F. Segal, Two Powers in Heaven and Michael Allen Williams, Rethinking "Gnosticism" (the last being an extremely strong book, in my opinion).

If you're interested in a more controversial take on the subject, there is Timothy Freke and Peter Gandy's The Jesus Mysteries (along with the follow-up book, Jesus and the Lost Goddess). This book essentially starts from the standpoint that Gnostic Christianity was the "real" Christianity, that it was a mystery-cult, and that essentially a bunch of idiots who weren't initiated into the inner mysteries then went on to deny that there were any inner mysteries, thus creating standard Pauline Christianity. It's written for the lay reader and raises a lot of very good questions, though of course I think they skew their conclusions a bit. Still, I highly recommend it as something to think about and something that can help an individual re-examine what he believes and why.

However, the point of all this is that if you're wondering why Protestants never "went back" to any of these texts, it's because they'd have to throw out pretty much everything that they believed in order to incorporate the books. No vicarious salvation, no Old Testament, etc. It would essentially be a break, not from Rome, but from the New Testament as we know it.