Sir, - I doubt if there ever was a theistic philosopher who thought he could prove the existence of God in a way that would convince everybody. Proofs can fail because they lack probative force, but also because, though they have such force, those to whom they are proposed are not able to appreciate it.
I like what Aquinas says: "It is natural for us (nobis naturaliter insertum) to know in a hazy sort of way (sub quadam confusione) that God exists." This natural knowledge of God's existence implicitly operates in every proof and attempt at proof of his existence; for every such proof and attempt rests on the innate metaphysics of the mind.
If the mind cannot validate itself, we can be certain of nothing. But such validation requires considerable acumen, philosophic training and experience, leisure and dedication; it is not a task for the tyro. It is one intimately connected with proving the existence of God. Anyone interested in these ideas will find them elaborated in, for example, Emerich Coreth's Metaphysics. The study concludes with the God question. Bernard Lonergan SJ was in substantial agreement with Coreth.
Lonergan himself offers what he regards as a comprehensive proof of God's existence: "If the real is being, the intelligible, God exists; but the real is intelligible, the real is being; therefore God exists" (cf his Understanding and Being for an exposition of the argument). Here again one sees that the proof for God's existence is bound up with the nature of the mind. What these philosophers are doing is demonstrating the truth of my quote from Aquinas.
The agnostic says, in effect, "I don't understand what it is to understand"; or, "I understand that better than do theists like Lonergan, Coreth, et alii."
A final point: a proof of God's existence mustn't be expected to tell us more about the divine nature than is warranted by the proof's terms and type of argument. In proving God's existence a proof may open only a small window onto His essence. People can be surprised to learn that Aquinas, having propounded his Five Ways - which, no doubt, he believed proved the existence of God - follows on immediately with the question "Is God a body?" - Yours, etc.,
M. Philip Scott O.C.S.O., Our Lady of Bethlehem Abbey, Portglenone, Co Antrim.