>>96404204
Check your history again. Constantine adopted Christianity after he reportedly saw the Christian symbol (that Roman soldiers wore on their shields after) in the sky before a battle that he won.
After that, most armies fighting the Roman army sees the cross of Christ in front of them. Fighting for one god or another was a new trend back then, especially in a large scale like an empire or kingdom, but its not something invented by germanic tribes. The Romans did it way before them.