This refers to
The First Council of Nicaea, I believe.
I am a great battle; long ago I took place,
It occurred in the 4th century AD.
The name is Nick though that wasn't always the case.
Nicaea is now called Iznik, Turkey.
The King or the prince, which one was greater?,
Debating over the primacy of The Father or the Son.
Some said "The king!, The prince, he came later!"
The presbyter Arius was in favour of sole primacy of God since Jesus came later.
In the end, the high king's faction lost,
The Father and the Son were affirmed by the Council to be worshipped on equal footing.
And the emperor banished the leader of their host.
Arius was banished by the convenir of the Council, the Emperor Constantine.
No blows were exchanged; not one single kill,
It was just a theological debate.
But my outcome is affirmed by millions now still.
Every weekend Catholics say the Nicene Creed, formulated and affirmed at this Council, during Mass.
The emperor was a real (i.e. not metaphorical) emperor.
Hint #1 Resolution:
The emperor Constantine was indeed a real emperor.
One line is much more helpful if understood phonetically instead of literally.
Hint #2 Resolution:
Nicaea and Nick sound phonetically similar.
The outcome is affirmed multiple times a month.
Hint #3 Resolution:
The outcome (the Nicene Creed) is affirmed every weekend at Mass.
I'm Not Relatively Inferior
Hint #4 Resolution:
This is a really amusing and helpful hint, and it’s pretty meta too (it’s what tipped me off to the answer!) The initials are INRI — short for Iesus Nazarenus Rex Iudaeorum (Jesus of Nazareth, King of the Jews) — which was written on the wooden sign identifying him as he was crucified. It also refers to the fact that Jesus was not relatively inferior to God, as the Council of Nicaea affirmed that he was to be worshipped as an equal of God as part of the Holy Trinity.