Having read a small amount of theology[1], I speculate that the broad and sometimes conflicting Christian perspectives throughout history make it difficult for the layperson to really grasp "true" Christianity (if there is such a thing.) Even going from say, St. Augustine to Kierkegaard, there's noticeable shifts in thought. I think the burden of proof squarely lies in advocates of Christianity, not the other way around.

[1] It's also been many years since I last touched the subject.