Imagine trying to explain to an ancient Greek philosopher how Christianity is monotheistic.

So, you have one God, but also a demigod, who is his son. There's a god of the underworld you call satan, and a number of divine or demonic creatures called angels and demons. Ok, throw in some ancestor worship in the form of saints.

This monotheistic pantheon would have sounded reassuringly familiar your ancient Greek interlocutor.