I replied, "Since the universe already exists, you either have to accept the universe itself as evidence or admit you are asking for evidence greater than the universe."
But has science has relegated God to the status of an unnecessary hypothesis in matters of cosmology - the study of the universe and how it came to be? Can an educated, scientifically-literate person agree with the Bible's proposition that, "In the beginning, God created the heavens and the earth?"
In classical terms, my conclusion is based on the "cosmological theory" of God's existence - that the universe, having come into being (having been created, in other words) must have had a cause. And it is entirely rational to call that cause the creator of the universe, for which a very good name is God.
Christians and Jews were not the only ones to come to this conclusion. Aristotle argued at length for the very same reasons. He called the universe's originator the "Unmoved Mover." Aristotle's reasoning was picked up by Thomas Aquinas and by Moses Maimonides, the greatest Jewish philosopher of the medieval period. Maimonides offered several arguments for God's existence, all of them some variation of a cosmological argument.
This argument is very powerfully buttressed by the discoveries and conclusions of modern-day astrophysics. Another argument in favor of God's existence is called the ontological argument. This argument attempts to prove God's reality basically by arguing that the term, "God," is nonsensical unless it has an existing referent rather than an imaginary one. Probably the most famous proponent of the ontological argument is Anselm of Canterbury, 1033-1109. Ontological arguments
proceed from premises which are supposed to derive from some source other than observation of the world — e.g., from reason alone. In other words, ontological arguments are arguments from nothing but analytic, a priori and necessary premises to the conclusion that God exists. In his Proslogion, St. Anselm claims to derive the existence of God from the concept of a being than which no greater can be conceived. St. Anselm reasoned that, if such a being fails to exist, then a greater being — namely, a being than which no greater can be conceived, and which exists — can be conceived. But this would be absurd: nothing can be greater than a being than which no greater can be conceived. So a being than which no greater can be conceived — i.e., God — exists.(Anselm's contemprary, Guanilo, argued in reply that Anselm's argument was absurd. As Anselm had cited the Psalm 14, "The fool hath said in his heart, there is no God," Guanilo snarkily titled his essay, "Reply on Behalf of the Fool," but though it was clever, it did not actually address Anselm's argument since Guanilo apparently misunderstood what Anselm was getting at.)
It is important to understand that Anselm said in his work that by "proof" of God's existence, he meant to demonstrate that it was reasonable to believe that God existed, not that is was therefore undeniable. In the twentieth century, Charles Hartshorne presented the ontological argument this way:
1) God can be analytically conceived without contradiction.
2) Therefore God is not impossible.
3) By definition God cannot be contingent.
4) Therefore God is either necessary or impossible.
5) God is not impossible (from 2) therefore, God is necessary.Now, in this argument, "necessary" does not mean "required." A necessary entity's existence does not depend on something else. "Contingent" means that the entity's existence depends on the prior existence of something else. In the ontological argument, God's existence is not dependent on any other thing. As Anselm pointed out, if there is an entity, however powerful, whose existence is contingent on something else, we would definitionally say that entity is not God. It would fail to be that "than which no greater can be conceived."
Non-contingency is one of the key points in the very idea of God. Over the centuries, many philosophers have tried to knock down the ontological argument, but, as atheist Bertrand Russell observed, it is much easier to say that ontological arguments are not valid than it is to explain why.
We reason that a non-contingent entity is possible. But we observe that contingent entities actually exist. The contingency of the things we encounter in everyday life is empirically self evident. In my undergraduate days, one of my philosophy professors observed that the cosmological argument and the ontological argument together made an extremely powerful case for the existence of God.
William Lane Craig recently put the cosmological argument this way, which I am paraphrasing. Suppose you are walking through the woods and come upon a perfectly spherical, transparent ball a couple of feet in diameter. You would not decide the ball was a necessary entity - that it existed without dependence on any other entity. You would immediately think it is a contingent entity, that its existence and location did indeed depend upon something else. "Who made it? How did it get here?" are entirely reasonable questions because it would be impossible to declare, with any sort of intelligence, that the ball simply popped into being in and of its own. No one would or could claim that the ball created itself. And, says Craig, if you expand the ball so that it is the size of the universe, the problem does not change.
So why are we so willing to assume that the universe itself simply "big banged" itself into existence when we deny that anything else does so? And if the universe simply appeared, just popped into being, why do other such appearances not happen routinely? Why, for example, don't spherical balls (or whatever) just pop up in my front yard or yours from time to time? But they don't.
In fact, the entire universe itself consists only of contingent events that bring forth only contingent entities. But somehow, we are nonetheless willing to say that the universe is necessary, that it exists independently of other another entity. I believe that it is neither intellectually coherent not congruent with empirical science to think that the universe, which consists solely of contingent entities and contingent events, is even so somehow itself necessary rather than contingent. No, the universe itself and entire must be seen as empirically contingent; its existence depends on another entity.
Since it is possible for God to exist as a necessary being, as explained above, the existence of the contingent universe means that God is also required. Therefore, God exists. However, I must side with Maimonides that arriving at this conclusion shows only that a creator exists, which we call God, but it tells us nothing about the nature of that God. For that more is required, which will be later.
A podcast of William Lane Craig's presentation, followed by two attempted rebuttals, is here, recorded at a formal philosophy conference.