Philosophical proof that 'God' is a man-made concept
To me, Plato's 'Euthyphro' still gives us the quintessential outline of why any God must have been made up by humans. The idea is that, unlike all other objective facts that we know about the world, for some reason it's up to humanity to specify all of God's attributes a priori, or by definition. Although Socrates was talking to Euthyphro about 'piety,' any suggested attribute of God can be equally called under question in the same way. We define 'the sun' to be something like 'the fiery ball in the sky,' but this definition is always open to revision based on further investigation in a way that the concept of God is not. And the (epistemic) difference between these two concepts is very important. Consider the following:
Many people think that our knowledge of God flows from God down to us through some religion or messenger or subjective experience. However, if this is true, then it fails to account for how we can recognize a religion as being from God in the first place. It fails for being able to decide who is a messenger of God in the first place. And it also fails to account for how we can determine which experiences are genuinely experiences of God and not just our own minds. If we are supposed to choose a religion based on how well it reflects God's will, then we must have some sense of what God's will is in the first place. Yet, if we learn what God wants from religion, how can we decide which religion is from God? It's like trying to decide whether a certain handwriting belongs to a certain person before knowing what that person's handwriting even looks like.
If choosing a religion is an arbitrary process that does not depend on our choosing a good religion, then the only possible standard for choosing between religions is one that man makes up. If it does depend on our choosing a good religion, then once again man must have some sense of what is good and what is bad prior to choosing his religion. In this case, again it is not what God wants that matters, but what the dictates of man's morality tells him to choose.
So ultimately humanity must be the final judge as to what religion he chooses and by what standard he chooses. To say that humanity must abide by God's standard when choosing a religion is to put the theological cart before the horse, since it presupposes that we already know what God's standard looks like, which is the very thing we would want to figure out!
Morality is a collective social construct, but it surely does not 'come from' religion. For if it did, then to say 'my religion is good,' would be merely tautological, since any religion would itself be (by definition) the source of good for its followers and would thus set the standard of right and wrong. If this is the way we're going to speak, then no two religions can be compared, since for the followers of extremist Islam (for example) their religion defines what is good for them. If this is the case, then it means that they have been completely indoctrinated and convinced not to question what they have been told. Indoctrination is an arbitrary process, since any religion at all can be good by definition. It is when we all work with a common standard of good and evil that it becomes meaningful to speak of one religion being better than another. But then if we already have this common standard of right and wrong, why do we need religion? The answer is obvious to me: we don't.
With some modification, the question posed by Socrates in the Euthyphro is something like this: Is a religion good because it comes from God (in which case it's important to follow the 'right religion')? Or does God give us a religion because it is good (in which case all that matters is being good)? If it is the former, then we have no method to decide what religion to follow. If it is the latter, then we don't actually need religion, per se, so long as we recognize what is good about any one religion. In either case, God must be judged by man, not the other way around. For there can be no imperative for us to obey an unjust tyrant, no matter what claim he has on our creation or however powerful he is. Might simply does not make right, and a God that does not pass our standards of morality does not warrant obedience.