Historical Contingency and Belief
It is important to understand that Maxentius, the opponent of Constantine for the sole leadership of the empire, had already withstood two sieges inside Rome by other rivals for the control of the empire. He had a numerical advantage in troops and most observers believed Maxentius would stay within the walls of Rome and wait out his third siege. The decision to fight a pitched battle outside the walls of Rome decisively changed the equation and was riskier for Maxentius than choosing to withstand a siege.
What led him to mount an attack rather than wait is entirely speculative, but something did. The battle of the Milvian Bridge was decided decisively when a bridge created by Maxentius' engineers failed while his army was making a tactical retreat. Maxentius was on the bridge and was drowned.
With his drowning, Constantine won the control of the western empire. Constantine then issued the Edict of Milan along with the eastern leader, Licinius, ending all persecution of Christians. He then fought several wars with Licinius, until Licinus finally surrendered and was then executed.
If you believe Christianity is a divine religion, rather than focusing on the death of a poor man on a cross in Palestine in the first century -- you should be focusing instead on this sequence of events. For if God intervened to resurrect Jesus, it would have all been wasted if Maxentius hadn't drowned, or if Licinius had defeated Constantine. Thus, the Christian must believe that God is intimately involved in each action that takes place on earth. God must have planned for Constantine and his family to take over the Roman Empire. He must have chosen to have Maxentius' army lose and Constantine's to be victorious.
You must also believe that God caused the early demise of Julian the apostate, as he had re-instituted the various pagan cults as the favored religions of the empire. You must also believe that God kept the armies of the Moors from defeating the French. You must also believe that God kept the Turks and Mongols from defeating the Holy Roman Empire. You must believe that God has continuously kept Christianity in a special spot, but for 2000 years has yet to get more than a plurality of humans to accept his word.
This sets up a nearly infinite recursion that really seems to me to lead to either panentheism or atheism. I'm curious what both sides have to say about the problem of historical contingency. I believe that it is simply logically impossible to believe in a Christian God and not believe that he causes virtually every act on earth (panentheism), which would leave you with something like Spinoza's God. Yet of course the God of Christianity can't be such a God, since there are so many things on earth that are so awful, that Christians can't imagine a good God to have caused them.
To highlight this, look at the actions of Constantine's family.
Virtually all of them either were killed, or became emperor. To believe Christianity is divine you must believe that all of those executions were being ordained by God, for if they had not happened, Christianity might never have taken hold.
The alternative, I supposed, is to argue that God did NOT act to kill of rivals to the Roman dominate, and that Christianity triumphed because of its success in the marketplace of ideas. I leave that argument to be fleshed out by those who take it seriously. I do not.