So was Constantine a true Christian? Who really knows, and does the question even matter?
One of the most important things to remember throughout this unit is the concept of historical understanding-evaluating times and people based not upon some unrealistic standard, but upon how they related to people in their times. Sure, Constantine did some horrible things, but do those things disqualify him from being considered a Christian at all? Not really. This is also a guy who (as brought up in D Period) produced the Edict of Milan, a document declaring religious toleration throughout the Roman Empire. There are positives and negatives to everyone. You can disqualify Constantine from Christianity for reasons relating to his time period, but if you disqualified every person throughout history who did not follow the Ten Commandments perfectly, you would have no Christians in history (well, maybe one, I guess).
Remember the theme of the unit-"How one constructs reality." For Constantine, his reality was a mix of Christian ideals and pragmatic necessities, spiced with polytheistic traditions here and there. In his mind, satisfying all these needs would make him the best emperor, while gaining him favor with his God. Understanding him in that context helps us better understand not only him, but the history of this particular religion as a whole. And the more we understand how people construct their reality, the more likely we are to solve conflicts between competing realities in the future.