Why would God?

[Sword of Christ]

So you don't fully believe the bible?

It's a book containing a revelation from God with some history in it not a book of history and science. You would call this form of revelation "special revelation" and then there is general or natural revelation, this would be what the ancient Greek ancient philosophers got up to. You can come to understand the existence of God without a special revelation though you wouldn't much about his nature or his relationship to his creatures without one. God could have spoken to people outside of the Biblical fold but Christianity is a true universal revelation of all humanity and it isn't culture specific. Islam is meant to be well though I think it's a deliberately manufactured copy by Mohammed and it is more of a culture/ethnic identity you need to adopt it doesn't mold itself around existing cultures.