Prayer is an all-sufficient panoply, a treasure undiminished, a mine never exhausted, a sky unobstructed by clouds, a haven unruffled by storm. It is the root, the fountain, and the mother of a thousand blessings. It exceeds a monarch’s power . . . I speak not of the prayer which is cold and feeble and devoid of zeal. I speak of that which proceeds from a mind outstretched, the child of a contrite spirit, the offspring of a soul converted – this is the prayer which mounts to heaven. . . The power of prayer has subdued the strength of the fire, bridled the rage of lions, silenced anarchy, extinguished wars, appeased the elements, expelled demons, burst the chains of death, enlarged the gates of heaven, relieved diseases, averted frauds, rescued cities from destruction, stayed the sun in its course, and arrested the progress of the thunderbolt. In sum, prayer has power to destroy whatever is at enmity with the good. I speak not of the prayer of the lips, but of the prayer that ascends from the inmost recesses of the heart.
John Chrysostom, On the Incomprehensible Nature of God, Homily 5.44, 46, 57, 58, quoted in The Ancient Christian Commentary on Scripture, Mark, 154.