The old, old story:
Once upon a time all the Christians agreed. And then a wicked heretic by the name of Arius appeared and started teaching that the Son was a creature, and not God. The church had a big council at Nicaea and kicked Arius out. But Arius was sneaky, and kept teaching lots of people, and so for 60 years lots of Arians, semi-Arians, and neo-Arians kept opposing up, in fact the whole church turned out to be full of them. Luckily Athanasius had a big brain and knew the bible really well, and said we must stick with homoousios or we will all be Arian. Then Athanasius died, but the three Cappadocian amigos took up his sword and fought against all the 10,000 varieties of Arianism, until they won and had a big council in 381 and killed Arianism and everyone knew that God was the Three in one. Then they argued about the two natures of Christ.