Argument from the efficacy of prayer
Prayer is the act of speaking to a god or goddess, either mentally or out loud, to profess loyalty, express gratitude or ask for favors. If prayers are answered, this is evidence that God exists. A related argument is that one may come to know God directly though prayer. Skeptics reverse this argument and conclude that Gods, or certain conceptions of God, do not exist because prayers are not answered.
- "To God and to man, the answer to prayer is the all-important part of our praying. The answer to prayer, direct and unmistakable, is the evidence of God's being. It proves that God lives, that there is a God, an intelligent being, who is interested in his creatures, and who listens to them when they approach him in prayer. There is no proof so clear and demonstrative_ that God exists than prayer and its answer. "
- "I have experienced that when I pray, things tend to work out better [...]"
The New Testament says that prayers are answered:
"Therefore I tell you, whatever you ask for in prayer, believe that you have received it, and it will be yours."
The argument runs as follows:
- Prayers are observably answered
- Only God could answer prayers
- Therefore God exists
Most objections are focused on the first premise.
God is supposedly unchanging
George Carlin suggests the act of prayer seems a little odd for people with belief in a god who is supposed to be impassible, omniscient and omnibenevolent. Clearly an omniscient god would already be aware of your problems and know what you want. Ultimately, your request may be regarded by God as either good or evil. If it is good, then why would God not have granted your wishes already? If it is evil, then why would God ever grant your request?
"Now, you come along, and pray for something. Well suppose the thing you want isn't in God's Divine Plan? What do you want Him to do? Change His plan? Just for you? Doesn't it seem a little arrogant? It's a Divine Plan. What's the use of being God if every run-down schmuck with a two-dollar prayerbook can come along and fuck up Your Plan?"
Billions of people pray for various things every day. Many of the prayer requests are even contradictory. For instance, in a football game, often the players and fans on both sides are praying to win. If God answers prayers, which side should He choose? The side that prays loudest?
Unclear effect of prayer
Christians often state that God can answer prayers in one of three ways; "yes", "no", or "wait". This makes God no different from random chance. After all, when you pray it can either happen (i.e. God answers "yes"), not happen (God answers "no), or you have to wait to see if it will happen (God answers "wait").  For this reason, skeptics argue that prayers are not answered.
"Successful" prayers rarely have an unambiguous form. Apologists point to events like cancers that go into remission or people waking up from comas as evidence for the power of prayer. Yet cancer goes into remission and people wake up from comas all the time. How are we to tell the difference between cancer that healed naturally (or thanks to the presence of skilled doctors) and cancer that was cured miraculously? A commonly asked question is, "Why doesn't God heal amputees?" Live footage of a severed limb miraculously regrowing would be far more convincing as proof of the power of prayer. Yet such prayers are apparently never answered, or answered away from the prying eyes of meddlesome investigators.
Some people say that it isn't God's will to perform unambiguous miracles through prayer. However, if it's all just God's will then why pray? God will just do what he wants to whether you pray about it or not.
Scientific studies into efficacy of prayer have not found it to be effective. 
- "These findings are equivocal and, although some of the results of individual studies suggest a positive effect of intercessory prayer,the majority do not and the evidence does not support a recommendation either in favour or against the use of intercessory prayer. "
"If you could give some scientific evidence that prayer actually makes an organic difference, not just makes you feel better, that would be something to put on the table. The fact that its not put on the table shows that prayer is pretty much talking to yourself."
"So I've been praying to Joe [Pesci] for about a year now. And I noticed something. I noticed that all the prayers I used to offer to God, and all the prayers I now offer to Joe Pesci, are being answered at about the same 50% rate. Half the time I get what I want, half the time I don't. Same as God, 50-50. Same as the four-leaf clover and the horseshoe, the wishing well and the rabbit's foot, same as the Mojo Man, same as the Voodoo Lady who tells you your fortune by squeezing the goat's testicles, it's all the same: 50-50. So just pick your superstition, sit back, make a wish, and enjoy yourself."
With anecdotal testimony of answered prayers, apologists are effectively cherry picking successful prayers. They don't tend to have testimony of prayers that God answers with "no". In fact, prayers are so ineffective it is arguable that modern Christianity does not fulfill biblical signs.
God does not heal amputees
- Main Article: Healing of limb amputation
It's not that God ignores some amputees, it's that he ignores all amputees.
Some might say that God does heal amputees by divinely inspiring doctors and scientists to cure them, or through the "miracle" of modern medicine.  This, however, is easily refuted by Occam's Razor. God's inspiration isn't necessary for doctors and scientists to find ways to cure amputees. And why did God feel the need to bypass this kind of helpful inspiration for thousands of years and reduce the suffering only in the modern age? There is still no evidence of miraculous healing which could be attributed to prayer.
Improbably events sometimes occur naturally
It's important to remember statistical probability when considering supposed miracles. For example, let's say there's a disease that has no cure and let's say 999,999,999 people so far have contracted it but none have survived. However, let's say the billionth person to contract it does survive. This isn't a miracle; it simply means that the odds of survival are now 1 out of 1,000,000,000.
- "There is nothing that is less probable than a miracle "
- Main Article: Which God?
Many religions claim that prayers are answered. It is unclear which God(s) or which theology is proved by this broken compass argument.
- "Even if only men from the first Baptist Church of Harvester were able to consistently have their prayers clearly fulfilled, where they pray for stuff and we can verify it came though consistently, let's say, at an 85-90%, what conclusion can we reach? Is it because there is actually a God answering that? We don't know what the mechanism is and we have no way to investigate that currently."
- E.M.Bounds, The Possibilities of Prayer,
- The best optical illusion in the world!
- Deacon Jeff, Comment #4 on Better theological response to "why doesn't God heal amputees?", Sep 14, 2011
- Rebecca Newberger Goldstein, 36 Arguments for the Existence of God: A Work of Fiction, 2011
- Atheist experience 2016-7-3
- Why Won't God Heal Amputees?
- George Carlin on Religion and Prayer (full version)
- Proof #44 - Think about a Christian housewife from GodIsImaginary.com: "Take one moment to think about a typical Christian and her "answered prayers.""
- Prayer on An Encyclopedia of Claims, Frauds, and Hoaxes of the Occult and Supernatural (By James Randi)