Things that could have failed to exist cannot explain themselves, so reality rests on a necessary, self-existent being.

Everything we observe could have been otherwise or could have failed to exist at all. We call such things contingent and argue that a collection of contingent things cannot explain itself. It must finally depend on something that exists by its own nature and could not fail to exist, which is what theists mean by God. This argument, associated with Gottfried Leibniz, does not depend on the universe having a beginning. It also answers the objection “who made God,” because the point is that every worldview must stop at something uncaused. The only question is whether that something is the physical universe or a necessary mind behind it. Honest limit: the argument rests on a principle that everything has an explanation, known as the principle of sufficient reason. That principle is intuitive but contested, and an atheist may reply that the universe itself is the necessary, self-existent thing, or simply a brute fact. So this is a serious argument, not a proof, and its strength depends on how much weight the principle of sufficient reason can carry.