Maximus the Confessor

There are two ways in which the prayer of a righteous man is effective. The first is when the person praying does so by offering to God his works done according to his commands. Then the prayer is not just a matter of words, blurted out meaninglessly with the empty echo of the tongue, but powerful and living and inspired with the spirit of the commandments. For the true basis of prayer and supplication is the fulfillment of the commandments by virtue. This makes the prayer of a righteous person strong and full of power. The second way is when the person who asks for the prayers of a righteous man fulfills the works of prayer, above all by putting his life right. Then he makes the prayer of the righteous man strong, because it is reinforced by his own wonderful conversion. For there is nothing to be profited by a person who makes use of the prayer of a righteous man if he is himself already more inclined to virtue than to vice.