The Atheist will claim prayer is useless because God should prove Himself by giving the Atheist whatever he or she prays for.
Here's an example:
"If God is real, then I want proof by being given 6 winning lottery numbers."
This demontrates a fundamental misunderstanding on the part of the skeptic. Prayer is not the same as the Genie granting a wish to Aladdin, or Santa Claus giving children the toys they ask for.
Prayer is conversation. It doesn't necessarily involve asking for stuff. And when it comes to asking, there's no guarantee that the answer is "yes".
God is not interested in winning souls by giving away toys and money. God isn't "Wheel Of Fortune" or "The Price Is Right". This idea on the part of Atheists is insulting. God condescended to certain people because He had a purpose in doing so. If you want to see what God is truly looking for, just read the Book Of Job.
The Bible also mentions the times when the prayers of Jesus failed because of a lack of faith on the part of the beneficiaries (Matt. 13:56-58). If the recipient was skeptical the prayers for healing failed.
So let's say God lets the Atheist have the 6 winning numbers plus the Powerball. The Atheist may say "thank you Jesus" then proceed to blow that money on toys and a party hearty lifestyle until it's all gone, as most Lotto winners are prone to do. Remember the Parable of The Progical Son? Except the Atheist may say "God prove yourself again by giving me more" instead of begging for his Dad's forgiveness.
God's answer might be"Tough luck, Chuck, you had your chance. When you had the means, did you do anything to honor me past your little thank you? Did you bother to think I might have given you this money for a greater purpose? You think I'm a fool, yet you're broke once more and still headed for an afterlife of misery."
Atheists claim they'll only believe if they receive what they have no right to expect. Christians, receiving no earthly riches, are thankful for the grace given to them by a God who sacrificed His Child for the sake of those who had no right to expect anything.
Think about it, Skeptics.