Of course, tophat, you don't actually answer my question. If being a Christian only requires, at a minimum, believing Christ was a prophet, then all Muslims are Christians. And that doesn't seem right. And you also misrepresent the Arian controversy that led up to the Council of Nicea. It did not arise until rather late in the history of the early church - the favorite heresy in the early church was gnosticism, which denied Christ's humanity. And in fact, nearly all of the early church fathers affirmed Christ's divinity as well as his humanity.
You want to argue that the desire for uniformity in belief only arose after Constantine, and it is certainly true that this desire was more prevalent then. But even scripture shows a great earnestness in preventing false doctrine, and the arian doctrine denying the divinity of Christ was rejected by a council in Antioch in 264.
Moreover, it's always unclear what most Christians believed in the early church, since most Christians could not read or write. You would be correct in saying that many
bishops were Arian (in fact, a little over half were), but that doesn't necessarily mean that most Christians followed Arius.
For me it comes down to two arguments. First of all, Christ claims to forgive sins. But it's nonsense to forgive someone unless they have sinned against you. Therefore the people Christ forgave must have sinned against him. The only way this makes sense is if Christ is God. (And, in fact, the people of the time understood him to be claiming this when they heard him claim to forgive sins). But the central point of Christianity is the forgiveness of sins. So it must also be central to Christianity that Christ is God.
The second argument: Christ claimed to be God (there are other reasons to say this than the forgiveness argument). A man claiming to be God is either a con man, a lunatic, or God, since the claim is either true or false, and the person making the claim can either know it's false or not know it's false. In the first two of these cases, it doesn't make sense to claim that he's a prophet -- he must be less than that, either on a level with the man who claims he's a poached egg or a very bad man. In the third case, he must be more than a prophet, since he's God.
So I've given two arguments and a reason in this thread, and one in another thread, as to why I think a Christian must believe in the divinity of Christ. Do you have a counter-argument?
(facts come from "http://www.newadvent.org/cathen/01707c.htm". The second argument is from
Mere Christianity by C. S. Lewis (while I'm citing anyway
))