Catholic and Orthodox churches have a large body of saints each of which is associated with domains of life, people make offerings to the saints idols (paintings and statues) as you would in a polytheistic culture, small golden and silver trinkets usually. These practices were originally polytheistic practices that merely switched context with minimal changes.
Of course sacrifices which was both a Judaic and typical polytheistic ritual was dropped from both Hebrew and Christian practice.
Oaths is another one, oaths are a very polytheistic tradition, you swear to do something with the gods as the arbiter and the judge of you actions. I don't believe Hebrew tradition is very big on taking oaths, I think they actually forbid it, I may be wrong.
As another example the Pope is a pontiff, that is a roman religious office predating Christianity.
Point is when Christianity spread in the empire it evolved to include a lot of the religious practice of the people of the empire.