One fact every atheist must learn is that truth knows no borders. Sadly, a large number of them aren't aware of this fact, which is why they so often point out that specific regions tend to have their own dominant religions. This, in their view, is due cause to question the validity of those beliefs.
The first, and most glaring problem with this argument is that atheism itself is not evenly distributed across the globe. There are areas in which atheism is more popular, and areas in which it is significantly less so. Thus, atheism itself is subject to this argument.
Or at least, it would be, if the argument was valid. As it turns out, however, it commits what is called the genetic fallacy. The genetic fallacy occurs when a belief is criticised based on either its origins, or the perception thereof. I say the perception thereof because often, that perception is erroneous. There are Christians who were not raised Christian, or even in a Christian nation. For example, contrary to the common form of the argument, not everyone born in Pakistan is a Muslim. Many Pakistani citizens, including those who were raised Muslim, are Christians. So obviously, in those cases, the argument would be doubly absurd. You cannot, with a straight face, tell a Pakistani Christian "if you'd been born in Pakistan, you'd be a Muslim".
But even if an atheist was correct about the origins of a Christian's religion, that still commits the genetic fallacy, because the fallacy isn't incorrectly assuming the origins of a belief, but incorrectly assuming that those origins have any effect on its validity.
In my case, there is some credibility to the claim that my culture influenced my beliefs. I happen to live in a country that was strongly influenced by Christianity, and was even responsible for helping it spread throughout the world. I also have easy, and mostly unrestricted access to knowledge, and freedom of expression was a treasured value of my country until very recently. So yes, I both received Christianity from the culture around me, and had the power to verify it using epistemological tools handed to me by my culture.
Know what else I received from my culture? Literally everything else I know. Perhaps, if I had been born in another culture, I would believe the earth is flat. Perhaps I would believe Kim Jong Un is an omnipotent, omniscient god who can hear my thoughts and doesn't even need to poop. Perhaps I would even be an atheist. And therein lies the problem. Not only do the sources of my beliefs not affect the truth thereof, but it doesn't even explain why we absorb specific aspects of our culture.
See, a problem that even the Israelites had while God was personally commanding them through a pillar of fire is that there is no such thing as a monoculture. Even where cultures do have a clearly dominant view, dissent exists. Christianity here in England is declining. Fast. Islam, by contrast, is growing, and almost every child is taught, from a very young age, that Evolution explains our true origins. I certainly was, and for my entire childhood, I believed it.
So, how can I be sure that my religion is true, and not just a product of where I was born? Simple: Because it never was just a product of where I was born. See, when God designed our species, He gave us a gift that He only gave to one other creature: Complex reasoning. I can test my beliefs, and an amazing, almost unique thing about Christianity is that it actually commands me to do so (1 Thessalonians 5:21). This, in itself, sets Christianity above, say, Islam, which teaches that Allah is the best of deceivers, or Evolution, which teaches that the human brain is descended from the brain of lower animals. Christianity, unlike any other religion I know of (including other pseudo-Christian religions) teaches that human beings have both the ability and command to reason, and that we were given this by a Holy and omniscient God. We can logically have this discussion only because Christianity is true. And that's a glorious thing, not just because it means we can learn things, but because we can get to know God. But...
We have sinned against Him. The same God that commanded us to test all things and hold on to that which is good also gave us other commands. And we broke them. Each and every one of us has rebelled against God, and so He has pronounced judgement upon us. The punishment? Death. But we don't have to take it ourselves.
2,000 years ago, Jesus Christ, the Creator Himself, came to earth as a baby boy. He grew up, He lived a perfect human life, free of sin. He did not deserve punishment, as we do. But He received it anyway. He received it on our behalf. As a result, we no longer have to. We can instead be rewarded for His righteousness. How? By confessing Jesus as Lord, and believing He rose from the dead. It really is that simple.