This is an introduction. I will be going over this for some time, and I imagine many people will disagree with me, but... that is what makes things fun. You can argue with me, and then we can go have a beer afterwards, and when you least expect it I can slap a choke on you.
I am pretty cynical, but this is the way I see things.
Constantine is struggling to unite Rome. He has already had a fight to consolidate his own leadership, and now he has to unite a fragmented empire, that is facing threats on all sides. He sees one group of people, that are growing like weeds. Even though they are outlawed, killed and persecuted everywhere, their influence grows and grows.
What is even worse, is that most of them are pacifist, and will not fight for the empire.
Now here I am willing to withhold my cynicism a bit, perhaps the emperor had a legitimate encounter with God. But I do not believe that the Father of Jesus would have showed Constantine that putting a symbol on his shields would allow him to militarily conquer the world.
So whether Constantine converts to Christianity because of an encounter with God or out of political expedience (which I seriously suspect), he now has the task of making the Christian religion, the official religion of the Roman Empire. And here things really start to get difficult.
You see this incredibly expanding movement was also incredibly diverse. There were many different autonomous communities, many theologies, many different practices, and even many different accepted scripture texts. It was in truth and anarchic movement. How can you make this the official religion of Rome, when you cannot even define what this is? So Constantine, thinking like a worldly official says, we have got to organize and control this. And calls a conference of all of the "bishops" within the church.
I believe there is some truth to the idea that some people were left out for expedience sake. However, I do believe that a proper cross section of people were there. I also believe that the majority won the day. I believe the scriptures that were chosen were the more correct ones. I do believe that the creed was what most of them believed. However, because of what Constantine initiated, we now have sharp dividing lines between those that "have it right" and those that dont. There are those that are in, and those that are out? In addition, we move from Christianity being largely based on orthopraxy (proper living) to orthodoxy (proper belief). Since we now know what is the proper beliefs we must instil them.
What happens is Christianity then moves into a place of power, and influence. Its meeting places become places of honor, its priesthood people of power and honor. And the state, upholds the rules, and rites of the church. The church then gives moral character and approval of the state. And soon, in only a generation actually, people are being murdered for not believing the right things. In only a generation, St. Augustine pens the Christian version of Cicero's just war theory and Christians are fighting for the state.
Christianity has never fully recovered from this, and the current church systems are built upon a subverted foundation. Which is why I often say... tear it all down, and lets start from scratch.
We will flesh this out more later, let the fighting begin :)