Pagans / polytheists would respect other people’s gods. They would even worship them at times. They saw gods everywhere and in everything. One reason the Jews were seen as so weird was that they had one god, and refused to worship any others, but their culture was recognized as very old so the Romans respected their god as very powerful.
The Christians had the baptism ceremony, and part of being baptized is to forsake all other gods from this point forwards. Barbarians / pagans knew the Christian world as quite rich - the Vikings called Constantinople “the big city” and knew it as the place where they could sell all the cool stuff they stole during raids. A missionary would go to the barbarian leaders and tell them about their powerful Christian god, which was why the Roman empire(s) they fought were so grand and powerful. It was like a mind virus where one chieftain or leader would convert, win some battles, then assume the reason they won was their baptism. Because Christianity required ending worship of all other gods, they would aggressively convert the people under their control, and their nobles under them.
The Christians had the baptism ceremony, and part of being baptized is to forsake all other gods from this point forwards. Barbarians / pagans knew the Christian world as quite rich - the Vikings called Constantinople “the big city” and knew it as the place where they could sell all the cool stuff they stole during raids. A missionary would go to the barbarian leaders and tell them about their powerful Christian god, which was why the Roman empire(s) they fought were so grand and powerful. It was like a mind virus where one chieftain or leader would convert, win some battles, then assume the reason they won was their baptism. Because Christianity required ending worship of all other gods, they would aggressively convert the people under their control, and their nobles under them.