It is prayer alone that conquers God. Christ has determined that it should do no harm. He has imbued it with every power for doing good. And so it has no other function except to save from the path of death those who have fallen away, to build up the weak, to heal the sick, to cast out demons, to open prison doors, and to break the chains that bind the innocent. In the same way it flushes out sins, repels temptations, snuffs out persecutions, comforts the faint, delights the merciful, guards travelers, calms the waves, confounds thieves, catches those falling, and affirms those standing. Prayer is the wall of faith, the arms and weapons we use against the foe who stalks us from all sides.  So let us never walk unarmed.

Tertullian, On Prayer 29 (updated translation ANF 3, 691)