Arguments for God's existence, based only on knowledge of nature, any of them from the young earth creationists' casting aspersions on radiometric dating, to the denial that abiogenesis could have occurred without divine intervention, to the argument that the universe itself is so fine-tuned it must have been made by an intelligent designer, all of these arguments are hideously complex. I like to think that I'm almost a smart person, and I haven't been able to work out the math on any of these for myself.
I've been on one side or the other of these sorts of arguments, and I now find them dull, to the point of pain. I'm ready to say, "The earth is 6,000 years old, it was made on October 22, 4004 BC, life can't come from non-life, we live in an incredibly complex universe that couldn't have been an accident, and bananas were designed with human consumption in mind." I would say this just to escape from this sort of discussion the next time I'm trapped in one. I'd like to see what happens next. I suppose this would make the argument a lot dumber; I'd prefer to talk about whether a personal God can be known through impersonal nature over talking about abiogenesis and astrophysics and accumulated aquatic aluminum.
I suppose one could call this uncaused cause God, it's all semantics, that's fine. But, how does one make the jump from this uncaused cause to belief in Hashem or Allah or the Fates or the Triune God of Reformed Theology? When people ask each other, "Do you believe in God?" I don't think they mean, "Do you affirm that the universe results from an impersonal uncaused cause?" (Also, they rarely mean, "Do you believe in a divine essence who hates you?") Whenever I ask the question, I'm not directly looking for information or opinions on God, I'm trying to understand my interlocutor: the implied question is, "Do you try to relate to a god? How?" Nothing about causality or complexity or even beauty in the universe suggests that there's a big something that would care to interact with us.
So why do religious people try to demonstrate that their God can be known from observation of nature? Religious people who came to belief through mere study of the cosmos are rare. Most religious people were raised in their current religion. Most converts become converts not through a systematic exploration of alternatives, but through the influence of their friends.
I suppose that, for Christians in particular, the historicity of the resurrection is enough of a reason for belief for them. Likewise, Jews and Muslims would appeal to other supernatural historical events, the deliverance from Egypt and the giving of the Koran, respectively. (Non-monotheists tend to be less concerned with academic proofs that any God exists.) What stake do the religious have in the idea that God can be known from nature, if, in general, that's not how they themselves came to belief in God?
I first became interested in arguments about God and nature because I wanted to interpret the Bible literally; I didn't care much about proving God to other people, I was trying to reassure myself, that my beliefs about God were compatible with reality. I suppose that's one reason why I've had stake in the relationship between God and nature.
Another reason to talk about knowing God through nature is found in Romans 1:18-21:
18For the wrath of God is revealed from heaven against all ungodliness and wickedness of those who by their wickedness suppress the truth. 19For what can be known about God is plain to them, because God has shown it to them. 20Ever since the creation of the world his eternal power and divine nature, invisible though they are, have been understood and seen through the things he has made. So they are without excuse; 21for though they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their senseless minds were darkened.
It would be cruel of God to damn people without any warning. Jonah was sent to Nineveh not to evangelize, but to give a portent, like serving a legal summons or posting a notice on a condemned building. Instead of obeying God, Jonah fled to Tarshish because he didn't want the Ninevites to even have that warning. Likewise, those who believe that God is good and condemns some must assert that knowledge of what God requires can be found in nature.
This can be a beautiful idea, that God can be known from nature, because that means that regardless of race or culture or religious background, everyone can have some true knowledge about God; if this is the case, God is rather sporting and egalitarian.
I suppose that the strongest reason why religious people have stake in finding God in nature is that they already want to find him, period. Humans have a strong longing for a relationship with the divine. This attitude shapes our relating with everything.
People look at the Galle crater on Mars and see a happy face. Creatures with faces didn't move rocks into a face-shape to communicate with us. It looks like a face because whenever people see two dots over a line, we see a face. This is pareidolia, seeing patterns where there are none, simply because people look for patterns all the time. That people see a personal God in nature says more about people than God.
"I believe that, if a triangle could speak, it would say... that God is eminently triangular."
—Benedictus de Spinoza