Over on my "Nature is real, religion is illusion" post, today Phil asked a good question in a comment:
I've been pondering along some similar lines recently, so was primed to offer up some answers. But first, the question needs quite a bit of work -- as others already have noted in their own comment responses.
"God" is a meaningless word without a lot of elaboration.
To Spinoza, nature is God. To Plotinus, oneness is God. To Christians, God is a person (or people). To Hindus, God is universal consciousness (maybe...it's tough to pin down Hinduism).
So any proof that God existed would need to be highly specific if it was going to change the way I live my life.
I'd want to know such things as whether God was formless or had a form, whether God was conscious of us humans or unaware of our existence, whether God had any power to affect the universe or left everything alone, whether God has any relationship with us after we die or if death is the end of our existence -- to name but a few "what is God like?" queries.
Once I knew these things, I'd be able to decide whether God should have any effect on how I live.
This is an important point: it's up to us what meaning, if any, God would have (assuming we've gotten the persuasive proof of God's existence that Phil spoke about).
After all, there are plenty of important things that exist right here on Earth that don't have any impact on the meaning I ascribe to my life. Or at least, very little impact.
For example, I'm aware that Mt. Everest exists. But I have zero interest in climbing it. I know that Paris is a city in France. But I don't really care if I ever visit it. Other people, though, are deeply interested in climbing Mt. Everest and/or visiting Paris.
Meaning doesn't reside in an object or person, but in our reaction to, or relationship with, that entity.
Thus depending on what sort of God divinity turned out to be, I might be attracted, repelled, or indifferent to him/her/it. He/she/it might make a big difference in how I live my life, a small difference, or no difference at all.
Since I have more than a minor interest in what, if anything, will happen to me after I die (nothing being the most likely answer, since that is what I'll likely be myself), if I knew that God would reward or punish me in an afterlife based on what I did in this life, for sure I could be persuaded to act in different ways depending on what was in it for me.
Ideally, of course, I'd get proof that God just adores people who are devoted to inquiring into the nature of existence via a churchless blog, and will reward them (namely, ME) with hot sex, pleasurable drugs, fawning acolytes, an endless supply of high tech gadgets, and flawless health/good looks.
Getting all that after death would be cool. Getting it now also, even better (God, are you listening?).
However, the chances are slim that anyone, including me, ever will have proof (1) that God exists, or (2) what God is like or might want of us. Thus Phil's question isn't one that needs to be taken very seriously.
Regarding the second part of his query, what difference would it make if we knew beyond a shadow of a doubt that God did not exist, I have even more trouble coming to grips with this possibility.
Because it doesn't seem to be possible.
As I'm fond of saying, and am about to say again, there's no way to prove that something doesn't exist. (Including, of course, irrefutable proof that what I just said is correct). It could always show up unexpectedly, appear out of the blue, manifest in a surprising fashion.
So I have zero expectation of getting proof of God's non-existence. And next-to-zero expectation of getting proof of God's existence.
All told, then, God has extremely little potential effect on my life.
Which is as it should be. Because my meaning comes from me, not God, just as your meaning comes from you.