Nonbelievers often object to quotes from the Bible as being used as an implicit proof, i.e. "it's in the Bible, therefore it's true." The verses quoted in this article are not intended as implicit proofs. However, since Christianity concerns the God of the Bible, it's valid to look at what the Bible says in reference to God and prayer when discussing praying to the Christian God.
If you object to Christianity on ethical grounds, you know that there is a big difference between believing that God exists and worshiping him. Praying to God and receiving a response would not only provide you with reason to believe in his existence, but would allow you to know something of his character as well - it can give you more information about who God is. Continued prayer - asking God to reveal himself to you, to help you understand the Bible, to teach you who he is - will develop a relationship with God in which you know who God is through your experience of him, and not just who he says he is in the Bible.
Why is knowing God's character important? Christians run across the same problems that atheists and skeptics do - evil in the world, God's acts of judgment on people in the Bible, etc. - yet their response is to continue to trust in God and worship him. What makes the difference is that Christians know God personally; they have a relationship with him and experiences that have shown them that God is good and trustworthy. This provides them with reason to continue trusting God even when they don't understand the reasons for everything he does. On the other hand, people who don't know God only have the Bible as a source of information; they've read about God, but they haven't communicated with him.
What sort of information can you learn from prayer? First, if you get a response, then you know that there is a God who is willing and able to communicate with you. An omniscient God knows what sort of response will make sense to you and be convincing to you; receiving such a response can also serve as evidence that God knows you well enough to know how to respond to you. Asking for help and receiving it, particularly when the help is not something that would ordinarily happen or that you could bring about on your own, is evidence of God's loving character. (Asking for things and not receiving them often turns out to be evidence of God's good character as well - Christians often realize they're thankful that a particular prayer wasn't answered. :-) Finally, God can teach you by enabling you to realize things you hadn't realized before, or to see things in a different light.
Why would prayer be effective?
God is capable of answering our prayers, both those that involve physical circumstances and those that involve our emotional state:
I know what it is to be in need, and I know what it is to have plenty. I have learned the secret of being content in any and every situation, whether well fed or hungry, whether living in plenty or in want. I can do everything through him who gives me strength. (Php 4:12-13)God knows us completely (Ps 139:1-16) and knows precisely what we need, even before we ask (Mt 6:8). God's omniscience also means he knows the best way to answer our prayers, including the best way to teach us and give us reason to believe that he exists and is good.Do not be anxious about anything, but in everything, by prayer and petition, with thanksgiving, present your requests to God. And the peace of God, which transcends all understanding, will guard your hearts and your minds in Christ Jesus. (Php 4:6-7)
God is willing to answer our prayers and encourages us to ask for things in prayer:
"Ask and it will be given to you; seek and you will find; knock and the door will be opened to you. For everyone who asks receives; he who seeks finds; and to him who knocks, the door will be opened.And God does in fact answer prayer. Personally, God has answered my prayers in ways ranging from granting me peace when I was distressed (and unable to make myself feel better) to enabling me to solve problems at work and in Bible study that I wasn't able to solve before. Biblically, God has answered many kinds of prayers (Acts 4:24-31, 12:1-17, Is 38:1-6, 1 Sam 1:1-20, Neh 1:1-2:8)."Which of you, if his son asks for bread, will give him a stone? Or if he asks for a fish, will give him a snake? If you, then, though you are evil, know how to give good gifts to your children, how much more will your Father in heaven give good gifts to those who ask him!" (Mt 7:7-11)
Why pray to an omniscient God who has already planned what he'll do?
God often decides to act based on whether or not people pray. This doesn't mean that God is dependent on people praying in order to do what he wants (if God wanted to take a particular action in response to prayer, he could see to it that there would be someone who would willingly pray at the right time), but that he chooses to respond to our prayers.
There are several examples in the Bible where God has acted in response to prayer and would have acted differently had no one prayed:
Why would the God of the entire universe listen to me?
Some people picture God as being constantly pestered by the prayers of millions of people, and wonder why God would even listen to them. Yet even with millions around the world praying to him, God is willing and able to respond to us:
Do you not know? Have you not heard? The LORD is the everlasting God, the Creator of the ends of the earth. He will not grow tired or weary, and his understanding no one can fathom. He gives strength to the weary and increases the power of the weak. Even youths grow tired and weary, and young men stumble and fall; but those who hope in the LORD will renew their strength. They will soar on wings like eagles; they will run and not grow weary, they will walk and not be faint. (Is 40:28-31)"Look at the birds of the air; they do not sow or reap or store away in barns, and yet your heavenly Father feeds them. Are you not much more valuable than they?" (Mt 6:26)
"Are not two sparrows sold for a penny? Yet not one of them will fall to the ground apart from the will of your Father. And even the very hairs of your head are all numbered. So don't be afraid; you are worth more than many sparrows." (Mt 10:29-31)
God treats each person as an individual. For some people, having God answer their prayer to know him would be the most effective way for them to come to believe in God. If a person's prayer to know God is answered, they know that God heard them and answered them - they've interacted with God and know from experience that God will listen to them and respond to them. Simply receiving evidence for God without asking for it first doesn't establish a relationship with God in the way that prayer does.
Asking God to reveal himself is a demonstration of one's desire and willingness to know God. If one is not willing to even ask God to reveal himself, one would most likely be unwilling to follow God no matter what evidence or revelation was provided.
Why hasn't God answered me yet?
No one can say for sure why God hasn't revealed himself to a particular individual. But let us consider the case of a hypothetical person, Aaron the Atheist. Aaron has prayed to God and not gotten an answer, or simply has never seen any reason to believe in God. Why not? There are many possibilities, a few of which are:
1. Aaron may need to go through some experience before being able to believe. Perhaps Aaron has no interest in God or religion because things are going just fine for him without God, and it's not until everything falls apart that he's willing to pay attention to the evidence that's given to him. Perhaps Aaron will believe in God when his prayer for his wife's health is answered and his wife is miraculously cured of cancer, but in order for him to be motivated to pray, it has to be his wife who falls ill (and thus he wouldn't have this evidence of God until after he got married).
2. It may be that Aaron's coming to believe in God is a process that doesn't occur instantaneously, but over a longer period of time. Changing one's beliefs from atheism to Christianity represents a dramatic shift in one's worldview; such a change usually doesn't happen overnight. Perhaps Aaron has prayed, and the first steps in this process have already occurred, but the steps are gradual enough that it's not yet evident to Aaron that his prayer is being answered.
3. It's possible that Aaron's attitude prevents his prayer from being answered. Faith in God is not required for a skeptic's prayer of "please reveal yourself to me" to be answered (see How can I trust God without knowing him first?), but at the very least, intellectual honesty and a genuine willingness to communicate with God is required. God told the Israelites, "You will seek me and find me when you seek me with all your heart" (Jer 29:13). As far as I know, God does not answer sarcastic prayers (e.g. "Okay, I'll believe in God if he suspends me in mid-air in the next five seconds...I'm waiting...Well, I'm still sitting down, so God must not exist").
4. It may be that Aaron is a staunch skeptic and nothing would convince him that God exists. If he saw a miracle occur, he'd insist there was a scientific explanation for it, or believe that he was hallucinating; if given further evidence that was convincing, he'd conclude that his emotions were getting the better of his reason, or come up with a reason why it still couldn't be true.
Why should I pray more than once?
Persistent prayer doesn't mean one should repeat the same prayer over and over like a mantra, rather that one should be consistent about praying each day. Of course God hears our prayers the first time we pray, but sometimes prayers can't be answered immediately (see Why hasn't God answered me yet?). One of the main purposes of prayer is to build a relationship with God, and the best way to do that is to speak to God daily, even if it's just saying, "God, I still want to know you, I'm still here listening for an answer." Continuing to ask God to reveal himself indicates a person's continued willingness to give God the benefit of the doubt and seek him.
How long do I have to keep praying?
If a person truly wants to know God, I believe God will enable them to find him before they give up completely. God works differently in each person's life; some people first pray when they are ready to become Christians, others pray for a long time before coming to know God. If you feel you've prayed for a long time without getting any answer, a question to ask yourself might be, "How would I respond if God did answer me?"
How can I trust God without knowing him first?
Some prayers do require the person praying to trust God first. Prayers for wisdom require that one trust in God's wisdom and ability to distribute it to others (James 1:5-8). Prayers for miracles require that one not doubt God's ability and not be simply testing God (Mt 21:21-22, Dt 6:16). However, this does not mean that unbelievers are unable to pray to God, or that they must agree to worship God without first knowing his character. All an unbeliever has to do is simply ask, "God, please reveal yourself to me."
Why does God want exclusive prayer and worship?
In an increasingly multicultural society, it may seem unreasonable that the Christian God wants prayer and worship to be directed to him alone. However, God does not insist on exclusivity because he's conceited, but because it's the right thing to do.
They pour out drink offerings to other gods to provoke me to anger. But am I the one they are provoking? declares the LORD. Are they not rather harming themselves, to their own shame? (Jer 7:18-19)If Christianity is true, then the Christian God really is the only God, and praying to another god is praying to someone who doesn't exist. Not only is that counterproductive, it's harmful: the person who does so is ignoring the help and advice that God is willing to give him, and is instead seeking help from a nonexistent deity. At best, he will receive no benefit from the false god, and will have damaged his relationship with the true God (for praying to a false god is really saying to God, "I don't trust you to answer me or help me, so I'm going to seek help from another source," and also, "I don't believe you when you say you're the only God; your words aren't trustworthy"). At worst, praying to a false god allows him to be deceived - for instance, he may fool himself into thinking that the false god has given him permission to do something evil.
How would I know if it's really God?
One can hypothesize that there's an omnipotent evil deity who occasionally answers prayers and claims to be Jehovah, Allah, etc. If that's the case, one has no hope of finding out the truth, just as one has no hope of proving that the world and its inhabitants weren't created last week and given fake memories. However, if Christianity is true, then God is the only God; and being both omnipotent and omniscient, he can make it clear to us who he is.
God hears us, wherever we are and however we pray (Ps 139:2-4, 65:2). We don't have to recite certain words or repeat them over and over to be heard (Rom 8:26, Mt 6:7-8). Prayer can be done standing, sitting, kneeling, eyes closed, eyes open, out loud, silently, or written down.
Prayers should be honest - there's no point in lying to an omniscient God (Ps 139:1-4). If you're angry, doubtful, afraid, etc., tell God how you feel. The Psalms contain several examples of this sort of honest prayer: Psalm 10, 13, 22. (Yet even the authors of these Psalms affirm that God helps them: 10:14,17-18, 13:6, 22:24.)
Prayers should be directed to God - the God of the Bible - and him alone (Ex 20:3-6). Commitment to God is an exclusive commitment, just as marriage is intended to be exclusive. (The idolatry of believers is depicted as adultery/prostitution in the Bible: Ps 106:38-39.) For a non-believer, prayer is comparable to dating with the intention of getting married in the future. Just as two people in such a relationship should insist on exclusivity, a non-believer investigating Christianity should pray solely to God.
What should a non-Christian pray? Nothing fancy is required; "God, please reveal yourself to me" or "God, please help me to know you" are adequate.