A family of arguments for God's existence from the existence and contingency of the universe. Classical versions: (1) Thomistic cosmological argument (Aquinas's five ways, especially the first three) — from motion, causation, and contingency to an unmoved mover, uncaused cause, necessary being; (2) Kalām cosmological argument (William Lane Craig, drawing on medieval Islamic philosopher al-Ghazali) — everything that begins to exist has a cause; the universe began to exist; therefore the universe has a cause; (3) Leibnizian cosmological argument — everything that exists has an explanation, either in the necessity of its own nature or in an external cause; the universe exists; therefore it has an explanation, which must be a necessarily-existing being (God).
The cosmological argument is one of the oldest and strongest theistic arguments. Four observations. (1) The Kalām argument is particularly strong given modern cosmology. Big Bang cosmology indicates the universe had an absolute beginning — a finite time ago, space, time, matter, and energy came into being. Anything that begins to exist requires a cause. The cause of the universe cannot itself be within the universe (space-time, matter, energy); it must be spaceless, timeless, immaterial, and personal (to initiate a first effect). This matches the biblical description of God. (2) Cosmological arguments face counter-proposals. Some physicists propose the universe could have uncaused quantum fluctuations, or that the multiverse removes the need for a cause. These proposals face philosophical and scientific difficulties — quantum fluctuations occur in space-time (so cannot explain space-time itself); the multiverse only pushes the question back (what caused the multiverse?). (3) Biblical grounding. The Bible assumes cosmological reasoning: "The heavens declare the glory of God" (Psalm 19:1); "since the creation of the world his invisible attributes... have been clearly perceived, ever since the creation of the world, in the things that have been made" (Romans 1:20). The apostles and Church Fathers have always held that creation points unmistakably to Creator. (4) Limit. The cosmological argument establishes that some necessary, personal, powerful being exists — it does not by itself prove the Christian God specifically. But combined with the historical evidence for the resurrection of Jesus, it identifies who that being is. The sequence runs: God exists (natural theology); this God has revealed Himself (special revelation in Scripture and Christ); therefore trust Him.