This argument wouldn’t undermine the Christian’s religious belief, specifically because it commits the genetic fallacy. This is the assertion that one’s belief is falsified because of the way one came to inherit that specific belief. In other words, would the religion of Islam be invalidated because of the way a boy came to hold it? Perhaps the boy is only a Muslim because he was indoctrinated into its system, but that would hardly tells us of the veracity and truthfulness of the Islamic religion.
There is also a counter objection here in the way that the atheist asserting things like this in the West would almost certainly not be an atheist if he were born in Syria, or Pakistan. Would a religious believer be in his right to conclude that atheism is false because the atheist was born in America? No, that conclusion doesn’t follow.