I think that the argument rest on confusion. The argument as I understand it go like this
- If God exist then his existence cannot be proved by reason
- God’s existence can be proved by reason
Therefore: 3) God don’t exist
Te argument supporting (1) goes like this:
i) If God exist then there is faith about his existence
ii) If there is faith about God’s existence then his existence cannot be proved by reason
Therefore: 1) If God exist then his existence cannot be proved by reason
Both are valid arguments, so any problem with them must be in the truth of the premises.
Lets start with (i). Why to believe such thesis? I see no reason for it. Just consider what faith is. faith is a subjective belief (a belief not supported by any evidence), or something of the sort, but if there is a God that created the universe then he existed before anyone believing in his existence (either beliefs based on proofs/evidence or not based on them). So premise (i) seems wrong, and it seems trivially wrong.
Premise (ii) seems false too, but is more interesting. Suppose I believe that there are aliens just by faith. Other people can believe in aliens by many reasons (for example, because is very unlikely that in such a big universe or ours only in Earth life developed). So there are faith about aliens’ existence, but also there are proofs/evidence about their existence. So there can be both. The same with God. Take San Ambrose for example, he had faith in God’s existence and he developed an argument for his existence as well, to convince others that didn’t have faith.
Finally, the more interesting philosophical thesis, the premise (2). Can it really be proved God existence? There are many arguments for God’s existence, but there are many arguments against those arguments as well. This is a very difficult topic, because the vary different arguments there are, and the vary different philosophical topics involved in them. For example:
a) What is a good proof of something existing?
b) What is existence?
c) What does our concept of God’s implies or contains?
d) Is it possible a proof of something existing a priori?
e) Is there such order in our world that someone have had to design it?
f) Are scientific explanations about our world sufficient to explain our world’s order?
an so on
I propose that the different arguments for God’s existence as well as their philosophical implications and the relevant questions should be taken seriously, and so, one by one in independent post.