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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.

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Edit: Although if you think about it holy communion is very sacrifice like in its mechanics.

Typically in a Greek sacrifice a food item I think typically some sort of animal protein was dedicated to the gods cooked on site and then split amongst the attendees, the gods are satisfied with the smoke as far as I understand.

Holy communion has very similar mechanics, food item then a ritual involving god, and then communally consumed.