The attendance of Constantine at the Council of Nicea was striking, but more striking than that was the existence of such a subversive council in the first place, and with an emperor’s knowledge and blessing. More about that in a moment.
It is often said that in his conversion Constantine was only seeking a unifying principle for his sprawling empire, and that his faith in Christ was not sincere. In some respects, whether or not his faith was sincere is beside the point, but I concur with Leithart that it was genuine. But what makes us think that the two motives are inconsistent? A man might come to Christ because he wants to save his marriage, but this does not make him a hypocrite.
So suppose for a moment that Constantine was not sincere. Why would such a man be tempted to turn to the Christians?
Another commonplace with regard to Constantine is that he was freaked out by the Arian controversy because it threatened to undo his realpolitik principle of unity right after he had adopted it. Right after he bet the imperial farm on the Christian horse, that horse got the staggers.
But why would Constantine have seen the Church as a principle of unity in the first place? The unity displayed by the Church, in the simple fact of such councils, was something that the ancient church had not really seen before. It was a new thing in the world. Rome had its Senate, but that was local, for the Romans. The Romans in turn ruled the world, but they did so in true top down fashion. The traffic flow of authority went outward from Rome. In the church councils, you see the development of a recognizable, universal representative form of decision making. It dealt with the things of God, true enough, but this was a senate gathered from over a huge geographical area.
So the Christians were not supplying something that the pagan religions had previously supplied, back before their sacrificial fires became impotent. The Christians brought something new. That idea was a visible demonstration of what true representative unity might look like. There is something enormously attractive to everyone in this. We see a secularist (and grotesque) parody of the idea in the United Nations. Whenever worldlings try to produce the fruit of faith on their own, without Jesus, the result is that Iran gets a seat on the body safeguarding women’s rights, and the Chicoms get to weigh in on the importance of human rights. The result, in short, is a sort of globo-joke. In other words, without Jesus, fuhgedaboudit.
The fact remains that the thing that Constantine saw was altogether lovely. “My righteousness is near; my salvation is gone forth, and mine arms shall judge the people; the isles shall wait upon me, and on mine arm shall they trust” (Is. 51:5). But in fact, it was the kind of loveliness that cannot be sustained for more than five minutes without Jesus Christ.
What Constantine saw was just a glimpse, a foretaste. It was a cloud the size of a man’s fist. The prophet’s words are being fulfilled, but they are not entirely fulfilled yet. We still await the latter rains — which are coming.