“Now, since the family and human society at large spring from marriage, these men will on no account allow matrimony to be the subject of the jurisdiction of the Church. Nay, they endeavor to deprive it of all holiness, and so bring it within the contracted sphere of those rights which, having been instituted by man, are ruled and administered by the civil jurisprudence of the community. Wherefore it necessarily follows that they attribute all power over marriage to civil rulers, and allow none whatever to the Church; and, when the Church exercises any such power, they think that she acts either by favor of the civil authority or to its injury. Now is the time, they say, for the heads of the State to vindicate their rights unflinchingly, and to do their best to settle all that relates to marriage according as to them seems good.”
— Vincenzo Gioacchino Raffaele Luigi Pecci, who became Pope Leo XIII. From his encyclical on marriage ‘Arcanum,’ 1880
Freegards
That is one hypothetical advantage that was had by the church ruling the state, as the combined early church did over Rome and then its Orthodox/Catholic pieces did long thereafter in respective territories.
But this exercise taught states that they could DO that, and then when no longer buttressed by the church, then came the follies.
Better that Caesar had never had that seat by any means, even as arm of church.
Anyhow the American answer (under the evangelical, popular model) worked as long as the significant We The People bothered to be godly. However, they carried governmental authority over marriage by inertia from the English monarchy, which in turn got it from the Anglican church which earlier had been, of course, the Catholic church, which in turn had been a piece of the original church.
Lose the godliness and the result is as tragic as the falls of ancient Israel. The structure is misused and the results hideous.