When Constantine took over the Eastern half of the Roman Empire from Licinius and founded his new capital at Constantinople in 324 AD, he was upset to find out that there was a big fight going on between the Christians in Western Asia. He had had about enough of that with the Donatists in
Africa! Still, he needed God on his side. Constantine's advisors told him that God wanted all Christians to believe the same way and not to argue, and so Constantine tried to get the Arians and the Athanasians (the two sides) to agree.
The Arians (the followers of a priest named Arius) believed that since there was only one God, Jesus was not exactly the same as God. First there was God, and then a little later He made Jesus. The Athanasians (the followers of a bishop named Athanasius) believed that there was absolutely no difference between Jesus and God. These differences might not seem important to you, but in the 300s AD ordinary people like you got into fights about this in the streets, and hit each other over the head with sticks. Each side had songs about how they were right, and they went around in big gangs of monks and priests singing their songs and getting into fights with the other side, especially in Alexandria, in Egypt.
Constantine held a big meeting for both sides in Nicaea (nye-SEE-ah) in 327 AD. He went to the meeting himself. But he could not get the sides to agree.
Like the Donatists, they kept on fighting for a long time. First the Arians would be in charge, then the Athanasians. They fought long after both Arius and Athanasius were dead. In the end the Athanasians won, and pretty much all the Romans converted to the Athanasian side by about 400 AD.
But meanwhile the Germans had all been converted to Christianity by Arian missionaries, so they were all Arians. It got so people thought of all Romans as being Athanasians and all Germans as being Arians. But one by one the Arian Germans were defeated or converted. Finally the last of the Arians, the Visigoths, converted to Catholicism (as it got to be called) under their king Reccared in the 590s AD.