I don't buy the idea shared by quite a few Christian apologists (usually of the fundamentalist variety) that every non-Christian is either (i) not well-informed about the relevant facts, or (ii) suppressing the truth in unrighteousness. Quite frankly, this is just stupid and overly simplistic. Christianity is not obviously true (although general theism is). In fact, it's clearly not obviously true. If you think the Trinity, the incarnation, the inspiration of the Old Testament, etc., are all obviously true, then I'm afraid you suffer from excessive and overly zealous dogmatism. Some may sincerely come to think that the probability of these doctrines is very high after much study, but that doesn't mean that every sincere person will. Some people may sincerely think they have successful arguments against Christianity--yes, even people who have studied Christianity for a long time.
That being said, in my opinion most critics of Christianity are pitifully underinformed about the relevant facts, and many critics of Christianity, like Richard Carrier, clearly have some underlying personal reasons for their views (I doubt any person as well-informed as Carrier would come to sincerely believe a proposition as absurd as <Jesus didn't exist> if it were not for non-rational reasons).