Here is the logic of that statement. God, by definition, is the greatest being that can be conceived. If He did not exist, He could not be the greatest conceivable being. Therefore, such a being exists. Stated another way, the fact that we have in us the idea of God demands that God is its cause.
Scientists generally agree that our world had a beginning.
And that beginning had to be contingent on a thing that was not contingent on anything else for its existence. Therefore, that noncontingent entity had to be infinite, eternal, everlasting, and self-existing. It had to be God. The purpose and design of the world point to the existence of God. Physicists marvel at the unbelievable complexity of everything they study.