What we need most (In my humble opinion) in looking at history--past or present--is more and more biblical knowledge. Here's why: because the more developed our understanding of the will of God, the better we are able to see and analyze the data of history. We need a highly organized sense of what "ought" to be, so that we can correctly interpret the complex, and hardly cut-and-dried nature of the "battleground of history" and the people we find there.
Instead of looking at Constantine as either "friend" or "foe" of Christianity, maybe it would be better to view him as simply a manager of the City of Man, within which geography the City of God was experiencing significant development. I think Constantine was an opportunist and a pragmatist, with very little religious knowledge.
Without postulating Constantine's state of conversion (genuine or not), I'm personally convinced that his rise to power represents a tactical change in Satan's strategy. Persecutions had not slowed Christianity's growth. Satan's influence could increase more rapidly within the church, and on a grander scale, if "counting the cost" could be made to seem "not too much" or even a "net asset." Instead of being a menace and agent of hostility toward the church, the State would now be seen as an agent of favor--and favoritism. Good relations with the church and even positions of influence within it would now be paths to worldly success.
But this tactical reversal would have strategic costs as well. A church free of persecution, able to gather (even summoned!) in a powerful voice-making body, could in the providence of God effectively put down the Arian heresy--even when the Arians boasted much political capital. The Devil is history's biggest loser. His tactics change as he seeks advantages here and there, but his strategy is hopeless. And God's is the victory.
As the church, we live in God's history. We "succeed" when we keep foremost in mind the cosmic struggle, and only secondarily the historic trail and the present circumstances. Constantine, in my view, was both weal and woe for biblical religion. Proverbs 21:1.
