The argument is sometimes given to the effect that Christianity offers assurances that prayer should work, and since it does, Christians should be healthier and wealthier than nonbelievers. Since they are not, Christianity is false. I call this the "nothing fails like prayer argument."
Prayer in the Christian tradition has two trajectories. The believer is expected to present his needs to God, but its primary purpose is for the believer to open his own inner state to God's correction. The "promises" concerning prayer have a condition, they apply only if the prayer is in accordance with the will of God.