It is not true that Christian practices are mostly pagan in origin. You do not need to look far to find the origin of the vast majority of what you would have seen in a medieval church service. You can see the common origin if you compare and contrast an Orthodox Jewish synagogue service with an Orthodox Christian liturgy. The first half of the Divine Liturgy and the synagogue service are virtually the same even in terms of the motions — the Torah scroll is replaced with the gospel book. Christianity owes far more to second-temple Jewish practices than it does to anything pagan.

It’s worth noting that “monotheism” is a modern word. It was not a word used by theologians of the pre-modern era. The Bible is insistent that God is in some sense “one” or “alone,” and that you don’t offer sacrifices to other gods. People just thought that other gods were demons. It made a big difference to this point of view whether a sailor was asking St. Nicholas to help him get home safely or asking a demon for help, in the same way it makes a big difference whether you call the local mob or the municipal police when you have a problem. The latter are quite analogous to an angel/saint because police don’t have power or make laws in their own right, they are just servants of a higher authority (ultimately, the constitution). The mob, like the demon in the Christian point of view, are in disobedience to that higher authority or in direct competition/rebellion against it. There’s a really big difference even if both paths end with armed men with guns coming to solve my problem.