I mean, it should be obvious to Christians today that Jesus repeatedly and consistently made a clear distinction between the “kingdom of heaven” and secular nations. Separation of church and state was absolutely one of the premises the early church worked off of as a teaching of Jesus himself. The idea of a “Christian Nation” is indeed antithetical to the very foundations of Christianity. The early history of Christianity is pretty lost on people today and people are quite unaware of just how drastic the changes in Christianity were in the early 300s, and how gigantic of a deal it was for the Roman state to adopt Christianity at the time.