(I don't think this is a good thing, just the way the world is:) People choose the religion they adhere based on what their parents and peers believe and have faith in (their interests also play a small role). This is why people who have christian parents and christian friends are mostly christian and why young people who grew up around atheists are mostly atheists. It's the same for Muslims, Hindus and other religions.
If Christianity were the true way then I assume there would be a lot of people without christian backgrounds and from places where Christianity isn't so common becoming Christians with the help of 'God'. But there are more
Christians converting to atheism and other religions than there are
non-Christians converting to Christianity. Most new Christians (of which
the vast majority are babies) are located in Africa where the local
communities are Christian and they don't have access to much outside
Additionally, what evidence is there Christianity or any other religion is true; they rely solely on 'having faith' (Abrahamic Religions at least).
The reason you are Christian is almost certainly because you were brought up with it, with peers and parents also following it. Had you been born in a Muslim country with Muslim parents, it's safe to assume you would be Muslim.
If God were the Christian God and that's why Christianity is the largest religion, why does he allow Islam, Hinduism etc. to be so big.