A standard response from skeptics against those who espouse belief in God is to pose this question: if God created the universe, then who created God? Here are a few reasons why it's unreasonable to ask 'who created God?'
Asking 'who created God?' is irrelevant to the original question.
Asking for a cause of a cause in no way disputes the reasonableness and efficacy of the initial cause of the effect in question. If we ask for the cause of a cause of every effect, we cannot know anything. We can easily rest with the knowledge of what cause is sufficient to create an effect, and we don't have to have all the answers to come to a reasonable conclusion. For example, suppose that during an expedition an archaeologist uncovers a sculpture of a human bust. What is the most reasonable inference regarding the origin of the sculpture? Isn't the most reasonable position that a person created the sculpture? Suppose the archaeologist is simply unable to tell us anything else about the artist who formed it. He doesn't know anything about the group to which this person belonged. He doesn't know if the artist was male or female. He doesn't know anything about the artist. Does this lack of knowledge of the final cause, in any way, cast aspersions on his inference that the cause of the sculpture was an artist? Not at all. Our subsequent knowledge of the details of the cause is all gravy. Our inference of the final cause of the sculpture as a human artist is quite reasonable despite our lack of knowledge of all the background details including the question of where the artist came from. A similar approach is taken in all the sciences. Asking for an explanation of the cause of an effect is reasonable. Asking for an explanation of the cause of the cause in order to entertain the theory that the cause generated the effect we're trying to explain is misguided.
Asking 'who created God?' exasperates an infinite regress.
When we look at nature, we see causes generating effects, and we see those effects turn into causes that produce other effects. The material world is a bit like an elaborate arrangement of dominoes lined up one behind the other. When Christians (and other theists) say that God created the universe, among other things, they are referring to the dominos (matter), the platform where the dominoes can be arranged (space and time), and the particular arrangement of the dominos (laws of nature). God is the best explanation of many aspects of the origins of the material world, but perhaps even more importantly, God is the best explanation and solution for the problem of an infinite regress.
The other aspects of this issue are important as well, perhaps even more so, but for the sake of staying focused on this very specific issue, consider the arrangement of the dominos. In a purely material world, every cause (domino) needs a preceding cause. And if we work our way backward in time, we arrive at the very first known material "domino," the first known material cause, and this cause has no known material cause. In other words, the first domino is a mystery. Even when we ask the first question that comes to mind, we are being irrational - what was the cause of the first cause? By now, it should be clear that in a purely materialistic world, there cannot be a "first" cause. The universe must be eternal. The material world must be eternal and there must be an infinite number of finite causes and effects and an infinite number of past events. In other words, there must be an infinite number of dominoes. But this confronts us with the problem of an infinite regress. We now have to explain how a closed system as our universe with the finite amount of matter and energy can possibly have an infinite number of finite causes and effects - a task that is impossible. And those who hold to the multiverse as an explanation are no better off - not only having to deal with the initial cause of this universe but having to explain the initial cause of the multiverse - the proverbial kicking the can down the road.
You see, God is the only way such a runaway infinite regress of finite causes can be terminated. Without an immaterial entity that can interact with and create material causes, even just the first one, we are left with an impossible problem to solve. Asking 'who created God' is, interestingly, a tacit admission to the realization of the problem of an infinite regress. Unfortunately, bypassing God as a solution, which is the best way to solve the problem, only exasperates the problem of an infinite regress of finite causes. It is ultimately far more reasonable to say that the cause of the universe is an immaterial source that is powerful enough to have caused the universe to come into being.
Asking for a physical cause of non-physical entities is irrational.
Have you ever thought deeply about purely non-physical realities? Have you ever thought about numbers? When I say "numbers," I'm not talking about numbers written down on a piece of paper. The writing is merely the representation of the abstract reality of mathematics. Suppose we had no paper on which to write. Would numbers cease to exist? No. We have this abstract, non-material reality. Now, what accounts for this non-material reality?
Where do numbers come from? Do they actually exist or are they fictional? These are questions with which philosophers wrestle. There are many options offered by various people to try to explain the mathematical world. However, the one option that no sane person may offer is that a physical process has caused the existence of mathematics. Why is this? Well, physical processes do not create abstract realities. This is why demanding a material cause for an immaterial reality is utterly irrational, and the same goes for God. When people ask for the cause of God, there is usually a tacit assumption that the cause must be material. And it's clear from our example, this is silly. If the person asking 'who created God?' is open to an immaterial cause, it should be sufficient to point out that if someone created God, then that someone would be God simply because, God by the theistic definition, is eternal. And if we found the cause of that God, would we stop there? Or would we keep asking 'who created that God?' We would be needlessly creating an immaterial infinite regress. Using the principle of Occam's razor, it should be enough to realize that once we've got a sufficient explanation, we need not needlessly exasperate the issue.
Who created God? Asking for any cause for the greatest conceivable being is irrational.
God, by definition, is the greatest conceivable being, which means that there can be no other being that has more great-making properties - more powerful, wiser, good, just, etc. Asking for any cause of God, the greatest conceivable being, implicitly assumes that there is a being greater than the greatest conceivable being, which doesn't make sense. It would mean that this greatest conceivable being is not the greatest conceivable being, which would break the law of noncontradiction. Upon closer examination, the question ends up being irrational. The greatest conceivable being has to be self-sustaining and self-complete not owing his existence to anyone or anything else. Therefore, it cannot possibly have a cause.
Asking for a cause of an eternal God is irrational.
God, as understood by the major theistic religions, is eternal, and indeed this would be one of the great-making properties of the greatest conceivable being. To clarify, God has and will continue to exist forever. It is impossible for God not to have existed at any point in the past and to cease to exist at some time in the future. This concept might be difficult to grasp by human beings since we are at present engulfed in temporalities. We see change, decay, birth, and death and so, experientially, it may be difficult to fathom eternity. However, conceptually, we can easily imagine something immaterial existing forever. This breaks no laws of logic and is fully compatible with other abstract realities (i.e. mathematics). Asking for a cause of an eternal entity is simply misunderstanding eternality.
Who created God? No one!
God is an eternal being. God has no end and had no beginning. As it turns out, it's a grave mistake to ask 'who created God?' For the reasons outlined here and by others throughout history, God simply is.