Having warned against the desire of human praise in almsgiving, the Lord now corrects the same fault in prayer. For prayer, though the most inward of works, can still be corrupted if it is turned outward toward display. Those who love to be seen praying have already chosen their reward, since they sought the eyes of men rather than the ear of God.
The command to enter the closet does not require a bodily place so much as an inward turning of the heart. For the true secret chamber is the conscience, where the soul stands before God alone. To shut the door is to exclude wandering thoughts and earthly distractions, so that prayer may proceed from a collected and sincere heart.
Vain repetitions are forbidden, not because prayer should be brief, but because it should be truthful. Many words do not move God, who already knows what is needed; they reveal instead a mind that seeks to compel by quantity rather than to trust by faith. Prayer is not given to inform God, but to form the one who prays.
The Lord therefore gives a form of prayer that orders all desire rightly. It begins with God, not the self: His name, His kingdom, His will. Only after the soul is lifted toward heavenly things does it ask for what is necessary for this life. Even then, daily bread is sought, not abundance; sufficiency, not excess.
Forgiveness is placed at the center, because no prayer can be pure while the heart clings to resentment. One asks to be forgiven in the very measure in which one forgives, thereby binding oneself by one’s own words. This is not cruelty, but healing, for it teaches that mercy cannot be received unless it is also given.
The final petitions confess human weakness. To be led into temptation is to be abandoned to it; therefore the soul asks not to be forsaken. To be delivered from evil is to be preserved from that which would separate it from God. Thus the whole prayer gathers the soul, humbles it, lifts it, restrains it, and entrusts it wholly to the Father.
The concluding warning makes explicit what the prayer already implies: forgiveness is not optional. The measure of mercy one shows becomes the measure one receives. In this way the Lord joins prayer to charity, so that the words spoken to God may be confirmed by the life lived with men.
Source: St. Augustine, On the Lord’s Sermon on the Mount, Book II, Chapters 3-6