Prayer doesn't make sense. We Atheists know it doesn't work. But The thing is, it couldn't possibly work.
I'm going to be arguing as if the Christian god actually exists here. It's purely hypothetical, I'm not arguing for his existence, merely trying to refine how he could possibly exist. Before I begin, let me lay out a bit of the view of the traditional Christian god so you can see what exactly what I'm talking about. First, god is all knowing. Second, god has a plan. The second principle pretty much follows from the first. If god knows everything, then he had it pretty much all planned out from the beginning. Essentially, to the all knowing god, all of existence from beginning to end would be a fact, not a process. Alright, now lets get to work with this prayer bit.
Incompatibility of prayer and god's plan:
Action: I pray to god my generic prayer: "God, please make x happen"
P1: My request coincides with god's plan and my request is granted.
P2: My request does not coincide with god's plan and my request is denied.
P3: My request does not coincide with god's plan, but he changes his mind to accommodate me.
The third possibility, P3, just doesn't make much sense. God would know everything that could ever happen, so he should not be capable of changing his mind. It also sounds rather presumptuous that a mere mortal would be able to change the mind of a being that knows everything. Effectively, your prayer might as well be "god, regardless of what your plans are, I want you to make x happen". This sounds like the ultimate in hubris to me. In any case, the real problem here is that while P3 disqualifies god's plan, both P1&2 disqualify prayer itself. Essentially, in the first two examples, the most important factor as to whether or not your request will be accommodated is whether or not it was already going to happen anyway. So say that I pray for a raise. Whether or not I get the raise would not depend upon my prayer but whether not not god planned for me to get a raise. My prayer was useless.
Absurdity of prayer to an omniscient god:
Simply put, god already knows what you're going to ask before you ask for it. Supposedly, he knows our every thought and action we will ever make while we're still in the womb. What could we possibly ask for that an omniscient god would not already know? Also, if you've ever worked in retail, you know there's a big difference between what a customer says they want, what they think they want and what they really want. Supposedly, god would know all of those things and hopefully provide us with what we really want/need despite what we ask for. Put another way, god should know what is best for us better than we ever could and therefore we should trust his judgement over our own when it comes to future events.
If a god answers prayers they must not be omniscient.
If a god is all knowing, then there is no reason to ask for anything, they already know.
In conclusion, if you insist that god is all knowing then prayer is useless at both ends of the equation. God knows what you're going to ask for, and he isn't going to change his mind about his plans just for you, so quit asking. I can't help but compare people who pray to a couple of kids continuously asking their father "will you take us to Mt. Splashmore." Except, this time daddy isn't going to give in, the kids are just wasting their breath.
Which one is it then?
Alright, so which one should the Christian salvage? You can only pick one; prayer that works, or an all knowing god. If you actually read your bible, you'll find plenty of instances where god seems like he does not know everything as well as plenty of instances where he seems to answer prayers. Genesis alone is full of several examples. The fact that we were punished for eating of the tree when god should have known what we would do. The fact that god seemed to not know where Adam and Eve were when he first came down afterwards because they were hiding. The fact that he had to flood the earth, leaving just Noah and his family because humanity turned out worse than he wanted. These are just a few such examples and there are plenty more. The mere fact that god could get angry or upset about an event which he knew would happen tends to suggest that things turned out counter to what he expected. Sure, I could get angry about the cat urinating in the corner, even if I predict it will happen again, god would know what is going to happen with perfect accuracy. Therefore everything would always happen just as expected. He cannot be upset about any of this because to him, all of existence is a fact. It is not something that is changeable, he knows it all from beginning to end. It is a single stationary point that he himself created. In the instant he created it, he set it hard as a fact... unless he really doesn't know everything. In my humble opinion, the bible seems to support a god who does not know everything.