Often people ask, " . . . but who or what created God?" In asking this question, they do not distinguish between contingent being and non-contingent being. St. Thomas Aquinas explains this distinction in his Proof From Necessity, which he presents in his Summa Theologica:
"That which does not exist only begins to exist by something already existing. Therefore, if at one time nothing was in existence, it would have been impossible for anything to have begun to exist; and thus even now nothing would be in existence, which is absurd. Therefore, not all beings are merely possible, but there must exist something the existence of which is necessary. But every necessary thing has its necessity caused by another. Now it is impossible to go on to infinity in necessary things. Therefore, we cannot but postulate the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity."
Aquinas maintains that all finite objects are caused by other objects in a chain of causes and effects. In philosophical jargon, finite objects and living organisms in the universe are contingent (dependent on a cause outside of themselves.) However, an infinite regress of causes and effects is inconceivable. Therefore, there has to be a non-contingent uncreated, uncaused source or creator of the universe.
For an additional discussion that touches on the impossibility of an infinite regress of causes, see the introduction and links to a summary of the Kalam Cosmological Argument in The God Question.