Say you're a believer, and you have a friend with cancer. Why pray about it? If your god is omniscient, then he already knows what you want and, by virtue of your friend's continued illness, has already elected to disregard your wants and desires for reasons he doesn't feel compelled to make clear. What is it about putting hands together and speaking out loud that will change his mind? Of course, if you believe in an omnipotent god who keeps up regular interaction with the physical world, then he's the one who gave your friend cancer in the first place.

He knows that you'll pray for something and he knows that he'll ignore your prayers and he knows that you're a miserable sinner who can't obey the arbitrary rules that he's set up anyway...

If you can't possibly understand why he thinks it important to, say, crash a coach full of schoolchildren in a highway pile-up or drown thousands of people in a tsunami or burn down a house full of people but leave a bible undamaged, then you'll never have any luck finding reason and logic in all this nonsense.

Just know your place. And give money.

Modern theologians do a poor job of explaining the purpose of prayer. It is not intended to be a process of "giving God his daily marching orders."

Praying for a sick person, for instance, is actually for the purpose of comforting the supplicant, reminding himself that his sick friend is in good hands. Praying for "our daily bread" is a reminder that our needs are being met and that we are old enough, as my mentor used to say, for our wants not to be all that important to us.

So it's probably true theologians do a poor job of explaining what your version of prayer is.

And to be precise, it is not "Thou shalt not tempt God", but rather the prohibition not to test God (Deuteronomy 6:16 in the Christian bible). Probably the most well known reference to this is when the devil tries to tempt (and so 'test') Jesus in Matthew 4.

Otherwise, regarding prayer, the Christian Bible suggests that prayer should be an ongoing practice - which J lays out nicely in #3.

"Thou shalt not put God to the test."

If I pray, I'm expecting an outcome and I will register whether the outcome happens or not.
Does God react to my prayer or not?
I'm putting God to the test by demanding something and expecting it to come true.

of perspective. If we accept (as the bible posits) that God is all-knowing, then sure, one could reasonably argue that prayer is unnecessary - silly even; why pray if God supposedly already knows what we need?

Well, if we assume that God is not irrational (in instructing us to pray), then we should conclude it seems, that prayer has a purpose. What purpose? There are a few common scriptural threads we could consider.

One thread is that of the problem of pride: "He is the potter, we are the clay", "God opposes the proud, but gives grace to the humble", etc. Lucifer's pride resulted in his being cast out of heaven: "I will make myself like the Most High".

IOW - God is God - and we are not.

The other thread is that of faith. Scripture is very clear that "the righteous will live by faith".

I find it interesting that Faith was the number one attribute that Christ recognized, and pride was the attitude most often condemned.

So then, what about prayer? Simply put, I see prayer as an exercise in both humility & faith; we recognize/admit our insufficiency while at the same time lean on His "all-sufficiency" - for everything that we need.

So you ask: "Ok, I've prayed about it many times; why doesn't God heal my amputated finger? Surely that wouldn't be too hard?".

Well, I can't speak for God, but it does bring us back to the matter of perspective - ours (temporal) versus God's (eternal). If it is true that our souls are eternal, a simple comparison of our mortal life-span against an immortal life (eternity), puts into sharp relief that this life on earth is but a short-lived vapor, and the way things are, are not the way they will always be. Scripture informs us: "For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known."

I like the way C.S. Lewis (noted atheist who became a Christian) describes this:
“At the end of things, The Blessed will say, “We have never lived anywhere except in Heaven.” And the lost will say, “We were always in Hell.” And both will speak truly.”

Maybe prayer is not a conversation with God or asking God for favors.
Maybe prayer is actually a ritual about transcendence, similar to meditation.

Empty your mind, feel the connection to the universe... and don't you dare hitting on the creator of the cosmos to do you a favor, he's got more important things to do.

Because god doesn't exist and isn't going to do anything anyway.

Next question.

You pray for Gods will to be done on Earth as in heaven. That is what he taught on the mountain.

Of the sect you belong to or follow and there are over 30,000 Christian sects in the world.