Domine, Non Sum Dignus (Lord, I Am Not Worthy)

This is the shortest prayer in the Roman Rite that everyone in the church says, and it is not the Church's composition at all. It is a quotation.

A centurion — a gentile, an officer of the occupying army — asks Our Lord to heal his servant, and when Christ offers to come to his house he answers: "Lord, I am not worthy that thou shouldst enter under my roof: but only say the word, and my servant shall be healed" (Matt. 8:8). It is the one occasion in the Gospels where Our Lord marvels at a man's faith.

The Church took the sentence almost unchanged and put it in the mouth of everyone approaching the altar. Almost. One word is altered: puer meus, my servant, becomes ánima mea, my soul. The centurion asked for another; the communicant asks for himself, and asks not for a servant's health but for the healing of the soul about to receive God.

In the traditional Roman Rite it is said three times, striking the breast, first by the priest before his own Communion and then again before the Communion of the faithful. The threefold repetition is customarily linked to St. Peter's threefold denial and his threefold profession of love afterwards — though the Missal itself gives no reason, and the older explanation is simply that what matters most is said three times.