When Christianity first developed in the Middle East, it was not considered the official religion of government. In fact, Christians were persecuted for their beliefs. However, over the next centuries, Christianity came to be identified with politics, and the power of the kings in Europe was intrinsically connected to their endorsement by the Catholic Church.
The fact that Christianity came to be entangled with politics was extremely positive to its spread. Freedom of religion as we understand it now became impossible, as the state could only allow people to believe in Christianity if they wanted to maintain complete legitimacy. The maintenance of this religion came to be understood as connected to the maintenance of the state and a particular regime.