26 Next there follows here: “Therefore, if you have brought your gift to the altar, and there rememberest that your brother has ought against you; leave there your gift before the altar, and go your way; first be reconciled to your brother, and then come and offer your gift.” From this surely it is clear that what is said above is said of a brother: inasmuch as the sentence which follows is connected by such a conjunction that it confirms the preceding one; for He does not say, But if you bring your gift to the altar; but He says, “Therefore, if you bring your gift to the altar.” For if it is not lawful to be angry with one's brother without a cause, or to say “Raca,” or to say “You fool,” much less is it lawful so to retain anything in one's mind, as that indignation may be turned into hatred. And to this belongs also what is said in another passage: “Let not the sun go down upon your wrath.” We are therefore commanded, when about to bring our gift to the altar, if we remember that our brother has ought against us, to leave the gift before the altar, and to go and be reconciled to our brother, and then to come and offer the gift. But if this is to be understood literally, one might perhaps suppose that such a thing ought to be done if the brother is present; for it cannot be delayed too long, since you are commanded to leave your gift before the altar. If, therefore, such a thing should come into your mind respecting one who is absent, and, as may happen, even settled down beyond the sea, it is absurd to suppose that your gift is to be left before the altar until you may offer it to God after having traversed both lands and seas. And therefore we are compelled to have recourse to an altogether internal and spiritual interpretation, in order that what has been said may be understood without absurdity.
27. And so we may interpret the altar spiritually, as being faith itself in the inner temple of God, whose emblem is the visible altar. For whatever offering we present to God, whether prophecy, or teaching, or prayer, or a psalm, or a hymn, and whatever other such like spiritual gift occurs to the mind, it cannot be acceptable to God, unless it be sustained by sincerity of faith, and, as it were, placed on that fixedly and immoveably, so that what we utter may remain whole and uninjured. For many heretics, not having the altar, i.e. true faith, have spoken blasphemies for praise; being weighed down, to wit, with earthly opinions, and thus, as it were, throwing down their offering on the ground. But there ought also to be purity of intention on the part of the offerer. And therefore, when we are about to present any such offering in our heart, i.e. in the inner temple of God (“For,” as it is said, “the temple of God is holy, which temple you are;” and, “That Christ may dwell in the inner man by faith in your hearts”) if it occur to our mind that a brother has ought against us, i.e. if we have injured him in anything (for then he has something against us whereas we have something against him if he has injured us, and in that case it is not necessary to proceed to reconciliation: for you will not ask pardon of one who has done you an injury, but merely forgive him, as you desire to be forgiven by the Lord what you have committed against Him), we are therefore to proceed to reconciliation, when it has occurred to our mind that we have perhaps injured our brother in something; but this is to be done not with the bodily feet, but with the emotions of the mind, so that you are to prostrate yourself with humble disposition before your brother, to whom you have hastened in affectionate thought, in the presence of Him to whom you are about to present your offering. For thus, even if he should be present, you will be able to soften him by a mind free from dissimulation, and to recall him to goodwill by asking pardon, if first you have done this before God, going to him not with the slow movement of the body, but with the very swift impulse of love; and then coming, i.e. recalling your attention to that which you were beginning to do, you will offer your gift.
28. But who acts in a way that he is neither angry with his brother without a cause, nor says “Raca” without a cause, nor calls him a fool without a cause, all of which are most proudly committed; or so, that, if perchance he has fallen into any of these, he asks pardon with suppliant mind, which is the only remedy; who but just the man that is not puffed up with the spirit of empty boasting? “Blessed” therefore “are the poor in spirit: for theirs is the kingdom of heaven.” Let us look now at what follows.
Source: Our Lord's Sermon on the Mount (New Advent)