Someone once asked me why, if God is omnipotent, there is anything that ‘cannot be’ – basically, why the rules about things like sin and uncleanness? Why cannot a person who has lived an okay life, yet made mistakes, be in the presence of the Eternal God? The doctrine is that “no unclean thing can dwell with God”. Since all people sin, even unknowingly, all are less-than-clean – and try as we might, we can’t reverse that, or scrub our own selves clean. So, why this law that uncleanness and God can’t mix? There are two parts to the answer – why God will not break His own laws or consider Himself above them; and what it might actually mean to be in His presence – how it would affect an imperfect creature, such as one of us, without the cleansing power of the Saviour’s Atonement.
1. Why God will not break His own laws/consider Himself above them
A good ruler, who wants us to have trust in Him
So, why is God not above His own laws? I think of a good king: he abides by the laws of his country/kingdom, even if he’s the one who made them, because he wants his people to respect and love him, and they wouldn’t if he decided he could do whatever he wanted, without regard to law – history gives us endless examples of rulers who didn’t follow this path and met the consequences. You could say, though, that because God is God, no one could or would oppose Him, and we certainly couldn’t start a ‘revolution’ against Him if we didn’t like what He was doing – even the thought of it is blasphemy, besides the fact that it’s impossible, and it creates the idea that God is limited by us. Well, the analogy’s not perfect; but when you think about it, people do rebel against God, all the time; most of His children do, in fact. The difference in the reality and my analogy is that God is not an earthly king who is able to be overthrown; the danger is not to Him, but to the rebels.

So (a) is that God cares for His children sufficiently that He wants them to have confidence and trust in Him, which allows them to keep His commandments, which will lead them to happiness and back to Him. Besides this, a ruler who considers themselves above the law, or changes it merely to suit themselves, is not a good ruler, and is either immature or power-hungry (or both).
Because there is no need
He is also not above His own laws because He is a God of order; His creations exist in order – that’s why there are physical laws which we express through physics theories and mathematical equations; laws which stand up to scrutiny, and whose effects can be observed time after time. Those things which we call ‘miracles’ are only miracles to us because we don’t understand the laws governing them. God does, and works by such laws. I think, personally, it’s because He delights to – most of us have had the experience of working by certain laws, such as those involved in physics or music or athletics, and doing something we know is going to work, and loving it because it again proves the perfection and awesomeness of that particular law that we followed. There’s a certain glee and satisfaction that comes from it. (Those, therefore, who say that miracles cannot be, and that He isn’t a God of miracles, deny that God is omnipotent, and understands and works according to eternal law; thereby denying that He is, in fact, God). So why would He go against eternal law – His own law – when it’s so perfect, and so completely accomplishes His purposes? It’s not even necessary.
So, God doesn’t break His own laws because (a) He is a good – perfect – ruler, who understands how to rule correctly, (b) He desires us, and all His creations, to have confidence in Him, and (c) He doesn’t need to, because He has complete mastery of everything – every law, every circumstance, every good attribute – and accomplishes all of His purposes through law.
2. What the presence of God means
When we speak of “the presence of God”, what does that mean? God has said that no unclean thing can enter into His presence. Why, beyond the reasons given above? (It’s a law, and He will not break or change it). Why is that particular law even in place? Because it’s a fact. (Obviously, but let me explain…).

We, as imperfect creatures, could not abide that presence. We literally could not stand in it. Although it’s impossible to really imagine what the presence of God is like, I think it helps to at least attempt it (to understand the answer to this question). Imagine a Being so full of light that He is light; it emanates from every part of Him so that you cannot, with mortal eyes, even look at Him – think of our sun; so pure that it hurts you, so full of love that your soul is pierced and you can’t stop weeping because your heart is overwhelmed; so beautiful that you cannot speak or move, or even think… go on like this, imagining this sort of a Being as best you can, and then try to imagine yourself actually in that Presence. Could you do it? Could you be there? I think it would break us in pieces; we might burn or shatter from the wonderfulness, the perfection, the terrible beauty and extreme purity of such a Being.

Now imagine someone else – someone who’s done some really bad things – being in that presence. Can you? It just doesn’t mix; doesn’t seem possible. Even if that person and you or I were sorry for what we’d done that was wrong; for all the petty remarks, selfish acts and tendencies, wilful rebellion, angry outbursts, childish sulking, lack of endeavour, etc. Even if we wished we hadn’t done it (and I’m pretty sure we would, then, if we didn’t already), it wouldn’t change the fact that we had done those things, and were muddy and imperfect in the presence of that purity and brilliant light. (Interestingly, when Zacharias, father-to-be of John the Baptist, questions the angel Gabriel’s announcement that he and his wife, both aged, would be blessed with a child, Gabriel replies that he stands in the presence of God, apparently to impress upon Zacharias his power and purity, and confirmation that he is a messenger of great authority. See Luke 1:18-19).
Conclusion
This law, then, is a result of actual reality – and changing reality would be changing eternal laws, by which God abides. But rather than being just a statement of fact, it’s a loving warning and invitation to avoid the certainty of being shut out from God’s presence forever, by following the path laid for us by our Saviour; the one safe path that will lead us to a point where we can endure that presence, and glory in it. Imagine, then, how it might be to be able to abide that presence; to be able to endure the glory and beauty and wonder of it. To be able because we had been made perfect by a Saviour, who took our inadequacies and sins and guilt and disappointments, and turned them into wholeness, because He is able and because we allowed Him to.
Were God to change that law, somehow, so that unclean things could dwell in his presence, it would make him dirty by association. The light from our visualisation would be diminished, its purity and terrible beauty less because it lacked fidelity; rounded and easy instead of sharp and certain. In short, he would no longer be God. That is awful to imagine.
I prefer a God who lives by His own laws; who loves us enough to move and change us instead of changing Himself so that we don’t have to; who knows that through Christ, we have the potential to become beings of light and beauty and goodness ourselves, and who in fact intended things to be this way, and created this earth for that very reason. I love a God who wants that for me; who has planned for it to be so; who is perfect and wonderful in every way. This is a God I can believe in and trust with everything.
Related: An excellent talk by Elder D. Todd Christofferson on freedom and justice. Watch a video highlight.