Ideas spread virally through culture, but how an idea spreads is irrelevant to wether or not it’s true (genetic fallacy). For example, the philosophy of metaphysical naturalism was spread through the cultural influence of communist countries, but it doesn’t necessarily mean metaphysical naturalism is a false, cultural invention. The only way to evaluate any idea is to examine the arguments and evidence that support it.

The cultural spread of religion is exactly what we would expect if the Bible is true. The Bible never hides the fact that religion spreads virally. God isolated Israel from pagan nations because He knew intermingling with them would cause Israel to adopt their false religions (Leviticus 18:3). Furthermore, Jesus commissioned His followers to spread Christianity virally by going out into the world to preach the Gospel. People have many different reasons for converting to Christianity (logical, emotional, cultural, etc.), but there’s no rational basis for thinking the viral spread of Christianity, or any idea, invalidates it.

"If there really were a God, we would expect people to be drawn to the true religion over all the others because its claims were supported by far better evidence, not that people would mirror their environment."

This argument is undermined by the fact that people don’t always formulate their beliefs based on evidence. There are many factors at play such as confirmation bias, social pressure, emotions, and wishful thinking. You can find skeptics who aren’t drawn to the evidence for a round earth, but that doesn’t make the idea false. Furthermore, the Bible claims that many people will reject Christ and follow the religions of their cultures, which certainly reflects the reality we find ourselves in (Matthew 7:13-14).