They have grown despite well-publicised and reasoned, effective critical analysis by opponents. They have grown despite heavy opposition, sometimes led by the power of the State. They have grown despite stunning blows, which would seem to outsiders to be fatal to their beliefs. Jehovah's Witnesses have taken in their stride failed prophecies of the end of the world. Mormons split , but did not fall, over the reversal of policy on polygamy.
Some beliefs seem very hard to eradicate.
Even this year, there are people who maintain that corn-circles could not have been created by human beings, even after they have spoken in person to the people who admitted creating the hoaxes. In view of the well-documented histories of thousands of beliefs and religions of all ages which persisted in the teeth of seemingly overwhelmingly strong evidence, then surely sceptics are justified in thinking that the burden of proof is on those who say Christianity is not just another one of the same. On the face of it, it could well be that Christianity may have been fatally flawed from the outside. After all, according to the New Testament , Jesus spent three years preaching as one who had real authority, raised people from the dead, drew big crowds, and achieved such fame that even kings were desirous of seeing him - yet very few Jews converted. This is a smoking gun which has to be explained away.