The first thing, and that is what worked for me, is that if "something" exists than it can be perceived somehow, directly or indirectly. "Ideas" about it can only be cultivated after this direct or indirect perception.
The trouble with "God" is that if it is what is usually understood by the word, it can't be an object like the others.
It is not something that exists, but something that makes existence real. So it would be, in logical terms, anterior to existance and being themselves. Truly, in all senses that we use in everyday life, God does not exist if it is to be what we understand by God: the very source of existence.
If God is this kind of "thing", than, of course, our usual ways of examining "things" do not apply. Even the tomistic ways of Aquinas above. At the very most, as many philosophers have done and was sugested by Papist, one could go back and back in this causation process to the ultimate cause, but that would be inconclusive in terms of this "god" is really anything but the "cause" of everything. It could be Christ, Alah or the singularity - and, by that method, the singularity is by far the best choice.
Our research has to be not in our ideas, but in our very existence.
Here comes then two insights, one of C.S. Lewis and one that is mine.
My insight was that atheists and theists agree in one thing: we and the world around us are made of pretty much the same stuff and follow the same rules.
Atheists look at the dead, consciousless matter and think that we are that. Theists look at themselves, see consciousness, values, reason and will and then say that the world is probably like that from inside too.
C.S. Lewis once said that of the natural phenomena, the "I" is the only one we can see from inside. And this "I", is precisely the subjective "I". All the other natural phenomena, we just see "from outside".
Now, if nature is all that there is, if our existence is not a miracle, if our existence is not an exception in nature, our existence is under the same "norms" of all existence. So we are either "subjectivitiless" like we see the other things, or "subjective" like we see ourselves, for we are no exception in the universe. Well, if the only phenomenon we see from inside, is, indeed, bearing subjectivity, we either concede that all the others do, like us, possess some kind of subjectivity *or* that we *are* an exception in the universe.
At that point, we could stop and we would have pantheism. The collective inner "subjectiviy" of the universe would be the only "god".
But then we do have the very convincing and strong arguments of scientists that matter is a "dead" thing, no consciousness, just inert in terms of having any kind of subjectivity.
And the we have us, subjective beings, yes, very much attached to a carbonic robot, but yet we can "see" that the "I" and the robot don't perfectly match. They are like symbionts.
So, this "subjectivity" is a "weakness" in the strictly materialistc point of view. At what point would absolutely inert sub-particles become self-aware? What property of matter would that be?
This provides for the *possibility* of God only and that is as far as rationality can go. But there is a catch.
*If* God is, being beyond everything we know did exist, the only way would be trying to communicate with it.
Truth is that the vast majority of "rituals" in most religions are well-understood psychological processes of self-convincing and even self-brainwashing. To engage in one of them is to risk to brainwash yourself to see what the ritual is meant to make you see.
Again we find a deadend. There is no safe or rational way for us to cross from here to "where" that "god" would be.
But if that communication was not from here to there, but from there to here? Could this "god", if he existed, communicate with us?
The first thing here, then, is to keep it clear that this communication would not be different from any other, that is, a dog communicates as a dog does, the same with a bird, with Jack or Phil. So the claim that God should communicate as we would like him to is *necessarily* to ask to be deceived. It is like saying that unless you prove to me that subparticles exist without mathematics I wouldn't believe they do. At least for now, even the weird images we get from subparticles can only be understood with high maths.
If God exists, "any way" of communication will not do. He probably would communicate in a very specific way, typical of him. But we don't know anything about him, so how would we know it is him?
There is only one way: asking. That's what I did. "Hey God, if you really exist, help me believe it." Now, of course, if he is truly God, he would know if we mean it or if we are just, like Pilates, making a rethorical question and turning our backs.
The consistent response of millions of people around the globe about this experiment is that God *does* reply, specially in that existencial level I mentioned above.
Tell your friend to try it, if he is really into empiricism. I know he is up to a great surprise.