One of the more harmful of theological sophisms in early church history was the equation of "church" and "kingdom": that somehow the kingdom of God was synonymous with a geographical region or a particular temporal entity (i.e., the Roman Catholic Church).
Whatever really happened to Constantine, in 311 A.D., he did true Christianity no favors by attempting to make it a state religion. Sacralism - "Christian" or otherwise - pretends to have the civil authority subject to the oracle or to the priesthood in power. Homogeneity was it; no options. Automatically, anyone living in the realm is assumed to be a subject of the state and a follower of the state's religion.
For example, I wonder what religion the ancient Mayan civilization practiced? duh...
It seems likely that Constantine did not just spontaneously pull his "vision" out of a hat:
"As early as the year 250 Origen was already hinting, broadly enough, that 'If now the entire Roman Empire should unite in the adoration of the true God, then the Lord would fight for her, she being still [the reference is to Exodus 14:14]; then she would slay more enemies than Moses did in his day.' This is a broad hint in the direction of 'Christian sacralism', the suggestion that it would be desirable to re-define the Church of Christ, to make it a society embracing all in a given locality, rather than, as it had been hitherto, a fellowship of believers."
True Christianity instantly found itself in a position of rebellion and sedition for its necessary refusal to bow to the whim of a state regarding something as fundamental as freedom of conscience.
I wonder why primitive followers of Christ did not fight like others fight to impose a kingdom in the name of a religion?