1) If someone tried to justify their Christianity on the grounds that they believed it because their parents believed it, that would of course be an astonishingly poor justification. In fact, it would be a version of the genetic fallacy (criticising or supporting a belief based on its origins rather than on its merits). So, if there were no grounds for Christian belief other than believing what your parents believe the case for Christianity would be weak, and sceptics would be entitled to point this out. But there are plenty of reasons that can be offered in support of Christianity
2) Who we learn our beliefs from does not really matter when assessing the rationality of those beliefs. It certainly seems to be reasonably likely that if you are a Christian then you learned the faith from friends or family: but nothing very interesting follows from that. We cannot assess any idea until we learn it from someone. When it comes to assessing a religion, it is better to explore it with someone who is both well-informed and actually believes and practices that religion. And while it is true that many Christians had Christian parents, it is also true that we learn many of our most important lessons from our parents. We certainly cannot dismiss our beliefs in fair-play or the value of hard work because our parents instilled them in us.
3) We need to evaluate arguments for faith on their own merits. It is true that people defend different religious faiths simply because they have different cultural heritages. But we should not conclude that “beliefs which we receive from our culture are unreliable.” This is much too sceptical. Some cultures tolerate totalitarianism, despotism and genocide. Western culture does not. Should we be intellectually neutral towards genocide simply because we have been raised to abhor it? Western civilization has developed and defended the concept of human rights. Should we be sceptical of all human rights claims because some cultures do not have the concept?
4) Yes, we inherit biases and preferences from our own family and culture. But why should I think that my upbringing or culture prevents me evaluating my my religious beliefs in a rational way. It is shameless buffoonery to argue that a Mormon in Salt Lake City, an animist in Southern Sudan, a priest of Amen-Re in ancient Egypt, and an atheist in North Korea all face the same obstacles when evaluating religious beliefs. Unless someone can explain what in my background prevents me from assessing the evidence for Christianity in a reasonable manner, I have no reason for abandoning my faith.
5) How did Christianity become the world’s largest religion having started out with just a handful of followers? It certainly wasn’t because people just accepted their parents’ beliefs without question. Also, if we broaden the discussion out to consider societies, then it is clear that Christianity crosses all cultural and ethnic boundaries – it is a global religion. In fact, in the past it was dominant in Europe, but that is no longer the case (presumably because people rejected their parents’ beliefs) and in fact it is rapidly growing in other parts of the world including many parts of Africa, Asia and South America (presumably again because people rejected their parents’ beliefs).
6) In fact, new believers often face terrible costs for rejecting the beliefs of their society or culture. The widespread persecution of Christians in many countries such as Iraq, Pakistan and Nigeria. John Allen in The Spectator claims that “the global persecution of churchgoers is the unreported catastrophe of our time.”
7) But isn’t atheism correlated with greater education? Aren’t the most enlightened societies the most secular? Unfortunately, the evidence for either proposition is far from clear. According to this survey (see here), it is lower among people educated to secondary level or above, but religious belief is slightly higher (and atheism slightly lower) among those educated to postgraduate level (compared to those educated to undergraduate level), although the differences are relatively small. Some evidence from the US suggests that levels of education are higher among religious believers (although not for all religious groups) compared to non-religious people.
It also seems that atheism (or at least being non-religious) is correlated with wealthier nations. Could it be that atheism is an accident of birth, not only because most atheists would have been Muslims if they had grown up in Islamic societies, but also because they happen to have been born in more materialistic societies?