Some folks who decide to pursue the critical/academic side of Christian origins are attracted by a class or a professor who opens up new avenues of thought. For others, it is a book that does this. The book that opened my eyes was Raymond Brown's Anchor commentary on John. The Fourth Gospel is (perhaps more than any other in the NT) an entirely different read pre- vs. post-naïveté. Opening Brown's commentary was something of an Alice in Wonderland experience for me.
Was there a book or author that invited you to critical thought? If so, what was it?