Ever hear of Saul/Paul? Constantine? Nicea? The New Testament replacing the Old Testament while pretending it's some sort of fulfillment of it?
It is far easier to take something and corrupt it while pretending to maintain it than it is to get rid of it and start from scratch. It's called going with the flow, and then you just steer the flow into the direction you want it to go ever so gently so that by the time anybody notices you're off course, you're already way off course.
With Fascism, you keep the church, you keep the king, you keep the the institutions of government and society going, but reducing them to mere props to cover what your true intentions are,,,