22 For whatever other words we may say—whether the desire of the person praying go before the words, and employ them in order to give definite form to its requests, or come after them, and concentrate attention upon them, that it may increase in fervour—if we pray rightly, and as becomes our wants, we say nothing but what is already contained in the Lord's Prayer. And whoever says in prayer anything which cannot find its place in that gospel prayer, is praying in a way which, if it be not unlawful, is at least not spiritual; and I know not how carnal prayers can be lawful, since it becomes those who are born again by the Spirit to pray in no other way than spiritually. For example, when one prays: “Be glorified among all nations as You are glorified among us,” and “Let Your prophets be found faithful,” what else does he ask than, “Hallowed be Your name”? When one says: “Turn us again, O Lord God of hosts, cause Your face to shine, and we shall be saved,” what else is he saying than, “Let Your kingdom come”? When one says: “Order my steps in Your word, and let not any iniquity have dominion over me,” what else is he saying than, “Your will be done on earth as it is in heaven”? When one says: “Give me neither poverty nor riches,” what else is this than, “Give us this day our daily bread”? When one says: “Lord, remember David, and all his compassion,” or, “O Lord, if I have done this, if there be iniquity in my hands, if I have rewarded evil to them that did evil to me,” what else is this than, “Forgive us our debts as we forgive our debtors”? When one says: “Take away from me the lusts of the appetite, and let not sensual desire take hold on me,” what else is this than, “Lead us not into temptation”? When one says: “Deliver me from mine enemies, O my God; defend me from them that rise up against me,” what else is this than, “Deliver us from evil”? And if you go over all the words of holy prayers, you will, I believe, find nothing which cannot be comprised and summed up in the petitions of the Lord's Prayer. Wherefore, in praying, we are free to use different words to any extent, but we must ask the same things; in this we have no choice.
23. These things it is our duty to ask without hesitation for ourselves and for our friends, and for strangers— yea, even for enemies; although in the heart of the person praying, desire for one and for another may arise, differing in nature or in strength according to the more immediate or more remote relationship. But he who says in prayer such words as, “O Lord, multiply my riches;” or, “Give me as much wealth as You have given to this or that man;” or, “Increase my honours, make me eminent for power and fame in this world,” or something else of this sort, and who asks merely from a desire for these things, and not in order through them to benefit men agreeably to God's will, I do not think that he will find any part of the Lord's Prayer in connection with which he could fit in these requests. Wherefore let us be ashamed at least to ask these things, if we be not ashamed to desire them. If, however, we are ashamed of even desiring them, but feel ourselves overcome by the desire, how much better would it be to ask to be freed from this plague of desire by Him to whom we say, “Deliver us from evil”!
Source: Letters (New Advent)