Here is an outstanding interview with Edward Feser. If you are not familiar with Feser, he has two books that should be read by everyone including atheists that rely on Richard Dawkins to tell them what to think about philosophy and God.
‘Proof’ is a loaded term, which turns on our understanding of what constitutes knowledge. There are knowledge claims that are rooted in inference, and are therefore on various levels of probability. Some arguments for God’s existence use this approach. The fine-tuning argument, for example, says that the universe is so intricately tuned for life, that it is most likely that a Fine Tuner orchestrated the cosmos. A different approach in terms of ‘proof’ in establishing the existence of God is by rational demonstration. This is found in the classical writings of Aristotle, Plotinus, Augustine, Maimonides, Avicenna, Aquinas, & Leibniz. Feser writes that philosophical arguments are still the most adequate approach to showing there is a God—the God of classical theism. The God of classical theism is immutable, immaterial, eternal, uncaused, omnipotent, omniscient, perfectly good, and can’t be compared to created gods that are part of the physical world such as Thor, Zeus, and others.
The primary classical arguments for God’s existence from pure reason are Aquinas’ Five Ways. We will only summarize them here, and take a snippet at the 2nd Way, while offering a few comments; it is enough for us to sum them up, and offer you some recommended reading, because a full defense of them would require a small book (at least). So here they are: Aquinas says we can know God exists from 1) movement and change 2) efficient causality 3) contingency and necessity 4) gradations of being, and 5) design.
These arguments can be found in Aquinas’ Summa Theologicae, First Part, a, Question 2, Article 3. The 2nd Way is about “efficient causality,” which is what is listed below. Efficient causality is to say that when you consider something like a tree, that tree has an origin and it came into being. The tree’s efficient cause is the acorn that fell to the ground in grew into the tree that it now is. Similarly, a child’s efficient cause is his parents. And so on. Here is what St. Thomas says about efficient causes and how they lead one to conclude that God must exist. There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible. Now in efficient causes it is not possible to go on to infinity.… Therefore, it is necessary to admit a first efficient cause to which everyone gives the name of God.
As Feser notes, “The real debate is not between atheism and theism. The real debate is between theists of different stripes—Jews, Christians, Muslims, Hindus, purely philosophical theists, and so forth—and begins where natural theology leaves off.”He adds that discussions of God’s existence must begin with natural theology (i.e., the practice of philosophically revealing on the existence and nature of God independent of revealed theology (Scripture).
. Ibid, 15.