The claim usually comes in one sentence: Christianity was voted into being at Nicaea. In 325 an emperor gathered bishops, they decided Jesus was God, and the faith we know came out of that meeting. The confusion is about order. The story puts the emperor first, the vote second and the belief last, so the belief looks like a product of the vote. In fact the council met because a priest in Alexandria had begun teaching that the Son of God was a created being. The bishops rejected that teaching and kept a conviction Christians had been confessing and writing down for generations. Constantine shaped the meeting. He did not supply its conclusion.
Knowing that answer is not the same as being able to explain it. The explanation needs four concepts, and each one depends on the one before. First, what Arius actually taught. Second, the word the council chose to answer him and what that word rules out. Third, the difference between a text, a belief and the vocabulary that states it, since each has its own date. Fourth, what an emperor can control in a church dispute and what he cannot. If you skip the first three, the fourth turns into a conspiracy theory.
The movements below take those concepts in that order. Each one adds a skill: stating an opponent's view fairly, defining a contested term in plain words, dating evidence by kind, and measuring political influence without denying or inflating it. The final task asks you to put the four together into an explanation you can give aloud, in order, to someone who doubts it.