If you aren’t familiar with the properties of God, perhaps the strangest, to us, is that God is unchanging. It follows necessarily from the fact that God is simple, that is, he is not composed of separable parts that are capable of existing independently. That follows from the fact that God is necessary, unlike us, who are contingent. Since God is necessary, he cannot be composed of things which are not necessarily together. And since God is necessary, he cannot change, because change means some part coming into being or ceasing to be. Since God is necessary (and has no contingent parts), there is no part of him which is capable of not existing. So far, OK, but how, then, does prayer work if God doesn’t change. What does prayer do?
It’s easy enough if you only consider our side of prayer, that is, how prayer changes us. But that’s not all prayer does. Prayer can change the world. We can pray for good things to happen, and God can answer our prayers with good things, if often (having to take everyone’s good into account) in ways so complex we don’t understand them until much later if at all. Or we can get immediate answers to our prayers, as in the case of miracles. How can that possibly work if God is unchangeable?
I think that it will be easier to give the answer if we first look at the fact that we creatures are able to interact with each other. C.S. Lewis mentioned, addressing the question, “since God knows what’s best, how can it make sense to ask him for anything?” He pointed out that the same problem applies to umbrellas. Surely God knows whether we should be wet, so why give him our opinion on the subject by opening our umbrella?
The answer to that question is that God has given it to us to take part in designing creation. This is part of a general plan of delegation which God seems to have. For a great many things, instead of doing things directly God gives it to us to do his work for him. He could feed the hungry man himself, but he gives it to us to be his feeding of the hungry man by us giving the hungry man food. You can see this in the analogy of the parent who gives his child a present to give to someone else; the parent could have given the present directly but the parent is incorporating the child into the parent’s act of generosity. Unsurprisingly, God does a far more complete job of it than human parents do. This is part of why people can ignore God; they see only the action of the people incorporated into God’s generosity and ignore the rest.
When God gives us these things by way of delegation, what happens is that we end up acting sort of like a lens to the sunlight. From our perspective, we don’t change the sun, but we do change how the sunlight affects earthly objects. By holding our hands up we make a shadow, but holding up a lens we concentrate the light on a place, with a prism we break the light into distinct pieces and make a rainbow. Real life is vastly more complex than just lensing the sun, but it works as a metaphor to show us how you can change the effect of the sun without changing the sun itself.
Prayer is the same basic thing, except we can’t directly observe it. By prayer we interact with God such that we change not God, but how his unchanging love for creation is expressed in creation itself. Prayer is like holding up a magnifying glass in front of the sun, shaping where the light goes without doing anything to the sun.