Christianity came from God. It did not come from man. God used men to write down his word but Christianity is not a man-made religion. And to respond to your last point, there is no such thing as a Christian country. There are Christians who live in countries but there is no such thing as a Christian country.
In the Old Testament, the Israelites were God's chosen people. They were the people God chose to be his own. But not every Israelite is a Christian now. A person becomes a Christian by realizing they are a sinner, who has sinned against a holy God. In order to be made back into a right relationship with that Holy God, a sacrifice has to be made. So God sent Jesus to be the final sacrifice for those who repent and believe. And now we are called to repent of our sins, believe in the finished work of Jesus Christ, and be reconciled to God. That is Christianity. No nation can do that. It is an individual faith and an individual repentance.