Something you'll hear religious people say is that "God exists outside of space and time." I find this claim to be problematic for many reasons.
Notice that this claim makes such a god's existence utterly unfalsifiable. And unfalsifiability isn't something to be proud of; it's the hallmark of a bad idea. If you can't test a claim, if you can't confirm or deny it in any way, then there's absolutely no reason to believe the claim. So I would argue that when a believer tells you that God exists outside of space and time, they're ultimately giving you a reason to not believe that their position is correct.
Doesn't this claim strike you as extremely convenient for the religious believer? Rather than pointing to concrete lines of evidence that god exists, when things get really tough for them, they can just fall back on the claim that he exists outside of space and time. How convenient for them, and how inconvenient for anybody who's looking for convincing evidence. I view this claim as one of many protective measures used by the religious belief to help them maintain their beliefs despite the fact that they're unjustified.
Plus, how could the religious believer possibly know that god exists outside of space and time? If that were the case, how could they even find this out in the first place? It's not like they can step outside of space and time through a portal, find god sitting there in front of them in a big throne like "Hey, what's up Steve?", and say "See? There he is. Outside of space and time, just like I told you." This is just another example of religious people pretending to know things that they couldn't possibly know.
You could say this about any extraordinary claim and it would have just the same amount of force. What if I said that I believe ghosts exist, and when asked to present evidence to support my claim, I said that they exist outside of space and time. Is this not just a cheap evasion? Is this not just a way to shirk one's responsibility of justifying their claims?
In what sense does something that exists outside of space and time even exist? What would such a thing be made of? Matter and energy are things that exist within our universe. If God exists outside of our universe, and if he's not made of matter or energy, then what is he even made of? Is he even made of anything? Does he have any composition whatsoever? If not, then how is such a thing that isn't made of anything any different from a thing that doesn't exist at all? There's a great Matt Dillahunty quote that's worth mentioning at this point:
"A God that doesn't manifest itself in reality is indistinguishable from a God that doesn't exist."
Dan Barker made a good point about this topic in one of his debates, vs Kyle Butt, if I recall correctly. He noted that decision making is a temporal process: Your thoughts on a subject that lead to your decision precede the making of the decision itself. So how could a thinking, decision-making god make decisions if it existed outside of space and time and if he truly was a timeless being?
I'm not a physicist, and I certainly don't pretend to understand the origin of the universe, but Stephen Hawking has argued—either in Brief History of Time or The Grand Design—that, because space-time was created at the moment of the big bang, it's nonsensical to speak of anything existing before the Big Bang; to even ask what existed before the Big Bang would be like asking what's North of the North Pole? If time itself originated at the Big Bang, then how could something have existed before the Big Bang that was responsible for creating the universe? The very nature of our universe and its origin seems to refute the idea that some god preceded it and designed it.
It also seems that the only kind of god that would truly exist outside of space and time would be a deistic god—a god who's nothing like the theistic God of the Abrahamic religions. If God did act in the ways described in the Bible or Qu'ran, then clearly he doesn't always exist outside of space and time, because he would've had to step into our universe or interact with it in some way in order to be able to do things like create our solar system, create life, and perform miracles like the parting of the Red Sea. So it seems that their holy books refute this claim.
And if we assume that God is an atemporal being, does this not create problems of its own? If God doesn't experience past, present, and future like we do, but instead knows everything that was and ever will be, many foundational religious ideas become untenable.
For example, if God already knew, before he created the universe, that certain of his creations would go to hell, why even create them in the first place? Would he not be essentially creating them just to burn? If he knew that this was the outcome, why not instead not create people who he knew would go to hell to spare them this needless suffering? He's in a pretty unique position to prevent harm that he's responsible for creating every step of the way.
Let's imagine, by analogy, that I'm a woman. I decide to give birth to a baby, but decide that I'll only give birth to it if, after it plops out of my vagina, it lands on a conveyor belt that moves it 10 meters and drops it into a pit of volcanic lava.
What kind of a sick, twisted person would I have to be to do such a thing? Why would I specifically design such an awful system, even if I knew exactly what the outcome of my actions and design would be? I could have designed it differently: I could have had my baby far away from the pit of lava. I could have pointed the conveyor belt in another direction. But I specifically designed this contraption so that it would point in the direction of the lava pit and drop the baby in there, even though I knew exactly what the consequences of my action would be.
I would argue that a god who can see the future and knows the consequences of his actions is similarly responsible for the suffering of his creations that are destined for hell; the one key difference is that, rather than experiencing a quick death in scalding lava, his creations will burn and suffer for an infinite amount of time. This is completely incompatible with the idea of a loving designer who cares about us.
The theist will usually step in at this point and argue that free will is what sends us into hell, not God. But this is to overlook the key point that God knew in advance exactly which people—exactly what percentage of his creations—would be destined to hell. To continue the analogy, let's say I ran tests on my conveyor belt contraption before construction, and found that exactly 50% of the babies will get dropped into the lava pit, while the other 50% will crawl off it and land on some mattresses and cushions that I have set up beneath it.
Even though I gave these babies the free will and the opportunity to crawl off the conveyor belt, I still knew beforehand that exactly 50% of them would end up falling into the lava pit. The fact that I gave them the chance to potentially avoid a horrific outcome that I'm responsible for designing doesn't justify the fact that I created this system in the first place with full knowledge of what the outcomes would be.
Let's think of some of other problems this creates. How about the problem of natural evil? Theologians typically tapdance around the problem of human evil by arguing that god has given us free will—but natural evil is something that we have no control over. Why would an atemporal god who knows the future create a world that would get inundated by tsunamis, shaken by earthquakes, and afflicted with disease?
He created this world, apparently knowing perfectly well that it would be flawed in these ways, and he nonetheless chose to move forward with this creation, despite the fact that he presumably would have the power to create a world free of these defects that wouldn't cause millions to die needlessly and miserably.
Faced with these observations, it seems that god either wanted to create a world in which millions die and suffer in these ways, or didn't care enough to create a world where people wouldn't die in these ways. So, from the perspective of the believer, he either actively strove to bring about this suffering, or is utterly indifferent towards it.
The latter case seems like the safer option for the believer, but if it's the case that god is indifferent towards that suffering, what are we to make of the claim that god cares deeply about us, loves us, and created this whole universe to have a relationship with us? The two claims seems contradictory and incompatible.
Theologians have lost plenty of sleep trying to grapple with this dilemma, but it's no mystery for the nonbeliever who views our planet as simply the aggregation of material leftover from the formation of our star. Why are there tsunamis? Because occasionally tectonic plates shift underneath a body of water, causing waves to radiate outward and sweep onto nearby masses of land. Why is there disease? Because we inhabit a planet on which pathogenic organisms try to invade hosts so that they can reproduce.
Many facts make little sense if you believe in the existence of a god who's outside of space and time, but these mysteries and contradictions instantly evaporate if you reject this idea.