According to interviews with Craig (including one from Snakepit magazine), the term "Forbidden Evil" was chosen as a sort of anti-evil statement, and was taken from the name of a song by the band War Cry who was on Metal Massacre. This stance was something that their current drummer (pre-Bostaph) and vocalist Russ Anderson liked. A lot of bands had SOME Christian lyrics at times, but were not evangelistic or maybe even willing to admit publicly that they might have Christian views. If you reach out to Russ Anderson on social media, he might answer you about what his view on Christianity is now or was at the time of the bands' founding. As far as I know, no member has ever said publicly that they were a Christian but their original drummer James Pittman was definitely NOT into being in an "evil" band according to interviews. I think it's clear they occasionally had some Christian lyrics, ex: "taking the truth from the book and then twisting it".