It goes back to the days of the Gospels. They’ve been choosing Caesar over God ever since.
Way before that... it isn’t merely a “Christian” thang. God warned the Hebrews that having an earthly king was not going to work well for them. But no, they were jealous of other kingdoms when it should have been the other way around, the other kingdoms jealous of them! So God let them learn the hard way.
Christianity made explicit a plan of salvation that always had been implicit. That brought other issues onto the scene.