Paul did not write 2 Corinthians 6:14-18 in order to get the churches to pretend that worldly people are actually the children of God. He wrote that passage in order to tell us to get the people who make no effort to live the Sermon on the Mount out of the church. The church in Rome has not done that since Constantine embraced the Church and the Church embraced Constantine. Yes, they have tried to make whole cities and whole civilizations Christian, but that cannot be done. Instead, they have churches with liturgies that people go through, but their lives are no different than the world. Usually those are a large percentage of attendees, far more than half, just like in Protestant and Orthodox churches because we refuse to obey the Bible.
When I say refuse to obey the Bible, I mean getting the sons of Belial, ,unbelievers, those living in darkness, etc. out of the church (2 Cor. 6:14-18) and refusing to make a way for the saints to provoke one another to love and good works and encourage one another in the assemblies (Heb. 10:24-25). We could start with doing that. Paul said to get the wickedness out from among you (1 Cor. 5), and John said the difference between the children of the devil and the children of God is apparent because the children of God love each other but do not love the world and obey Jesus’ commands (1 John).
There are a lot of opinions and rumors about Constantine, the Council of Nicea, and the events of the fourth century that changed Christianity to Christendom. Not only will you get the incredible story, with all its twists, plots, and intrigues, but you will find out how history is done and never wonder what is true again.
The Rest of the Old, Old Story 