 From what I learned so far about Islam is that the worst of all sins would be shirk. And shirk is when you associate other gods with God or even partners with God. When you dive into polytheism away from monotheism, that would be the worst of all sins. But even if you look within Christianity, you see, of course, that the first commandment is that you should have no gods beside God. And this is the main sin, the worst of all sins. Because once you start associating partners or other gods with God, you start making everything else into God. You start defying other things. You start shifting your priority. And instead of having God as your first priority, you start worshiping money, women, cars and what not.