"What we want to say on the basis of this material is that God is a self-existent being. That is to say, all of finite reality depends upon God for its creation, for its present existing, and for its future being. He brings it into existence, he sustains it in being, and it will remain in being so long as he sustains it and conserves it into the future. In other words, all of reality outside of God is shot through with a radical dependence. It is in existence only so long as God creates and sustains its existence. Were he to withdraw his creative power, the universe would be annihilated in a blink of an eye.
If God exists, then why does God exist? What is the cause of God? Once
we understand the concept of God as a necessary being, then you can see
that this is a question which is, if not meaningless, then at least
obtuse. It is sort of asking, “Why is it that all bachelors are
unmarried?” Nobody racks his brain trying to figure out why it is that
every bachelor you meet is unmarried. Why? Because the very concept of a
bachelor is that of an unmarried male. Similarly, the concept of God as
the greatest conceivable being is that of a necessary being. Therefore,
it is impossible for God not to exist. His non-existence is impossible.
God had no beginning, he depends upon nothing, he cannot not exist.
Therefore, the question, “Where did God come from?” or “Why does God
exist?” simply shows that the person asking the question doesn’t
understand the concept of a necessary, self-existent being."