This really should be asked as “How is monotheism different than polytheism?”
To Christians (as well as Islam, Hebrew tradition, Zoroastrianism, Tenrikyo, Shaivism and Sikhism) there is a belief that all the processes of the universe were caused or set in motion at least, by one all powerful being. The different beliefs differ on the motivations of this being and the ways in which this was done and what we are supposed to gain from it. What they all agree on is that, above everything else there is one supreme being that is responsible for everything else.
In polytheistic faiths there usually is a finite limit to individuals gods. Gods can be born and be killed and therefore are not outside of the nature of time, as the monotheists believe. They have powers that are separate and individual, which means that they are not “infinite” in nature. They are also separate entities and are divided, giving us the premise of “poly”-theism and multiple gods as apposed to the unity of a single “mono”-theistic God.
The other major important element of this concept is the idea of a theistic, cosmic medium in which God, or the gods, or anything else carries on its existence. As I said before, most polytheistic faiths believe that in some way their gods were born, either from some cosmic event or from other gods. This suggests there is was some medium in which all the gods are currently existing now existed before. They were not the first thing. Not only are not the first thing, but they are not the greatest things in existance since they are a part of a greater universe that they can not completely control and that they are subject to. The monotheists believe, however, that God was present before the creation of the universe, exists within and outside of the universe, controls the laws that govern the universe and will exist after the universe would hypothetically cease to exist. In this instance, God is synonymous with the cosmic medium that other gods would exist within.
Let’s look at it one more way, this time an analogy.
Imagine yourself as a microscopic spec, a single celled organism perhaps, floating around a giant ocean. Polytheistic gods would represent the fish. Some are beautiful, some are strange, some are dangerous, they all play a role in your little ecosystem though almost none of them give even the slightest notice of you. They share many similarities to you, but are much more advanced. Their power is limited, but is vastly more than yours. A monotheistic God in this case, is the ocean. It is boundless and unimaginable and the medium for all other existence. The difference is that the monotheistic faiths believe that God is sentient and aware of their existence, whereas the water in my analogy does not.
Note:The above article is the work and thoughts of Jonathan Kirk Davis, Christian, Raised Baptist, Believes in Big Bang and Evolution Christianity, a member of Quora. This was posted with Jonathan’s permission.