5 “Therefore, when you do your alms,” says He, “do not sound a trumpet before you, as the hypocrites do in the synagogues and in the streets, that they may have glory of men.” Do not, says He, desire to become known in the same way as the hypocrites. Now it is manifest that hypocrites have not that in their heart also which they hold forth before the eyes of men. For hypocrites are pretenders, as it were setters forth of other characters, just as in the plays of the theatre. For he who acts the part of Agamemnon in tragedy, for example, or of any other person belonging to the history or legend which is acted, is not really the person himself, but personates him, and is called a hypocrite. In like manner, in the Church, or in any phase of human life, whoever wishes to seem what he is not is a hypocrite. For he pretends, but does not show himself, to be a righteous man; because he places the whole fruit [of his acting] in the praise of men, which even pretenders may receive, while they deceive those to whom they seem good, and are praised by them. But such do not receive a reward from God the Searcher of the heart, unless it be the punishment of their deceit: from men, however, says He, “They have received their reward;” and most righteously will it be said to them, Depart from me, you workers of deceit; you had my name, but you did not do my works. Hence they have received their reward, who do their alms for no other reason than that they may have glory of men; not if they have glory of men, but if they do them for the express purpose of having this glory, as has been discussed above. For the praise of men ought not to be sought by him who acts rightly, but ought to follow him who acts rightly, so that they may profit who can also imitate what they praise, not that he whom they praise may think that they are profiting him anything.
6. “But when you do alms, let not your left hand know what your right hand does.” If you should understand unbelievers to be meant by the left hand, then it will seem to be no fault to wish to please believers; while nevertheless we are altogether prohibited from placing the fruit and end of our good deed in the praise of any men whatever. But as regards this point, that those who have been pleased with your good deeds should imitate you, we are to act before the eyes not only of believers, but also of unbelievers, so that by our good works, which are to be praised, they may honour God, and may come to salvation. But if you should be of opinion that the left hand means an enemy, so that your enemy is not to know when you do alms, why did the Lord Himself, when His enemies the Jews were standing round, mercifully heal men? Why did the Apostle Peter, by healing the lame man whom he pitied at the gate Beautiful, bring also the wrath of the enemy upon himself, and upon the other disciples of Christ? Then, further, if it is necessary that the enemy should not know when we do our alms, how shall we do with the enemy himself so as to fulfil that precept, “If your enemy be hungry, give him bread to eat; and if he be thirsty, give him water to drink”?
7. A third opinion is wont to be held by carnal people, so absurd and ridiculous, that I would not mention it had I not found that not a few are entangled in that error, who say that by the expression left hand a wife is meant; so that, inasmuch as in family affairs women are wont to be more tenacious of money, it is to be kept hidden from them when their husbands compassionately spend anything upon the needy, for fear of domestic quarrels. As if, forsooth, men alone were Christians, and this precept were not addressed to women also! From what left hand, then, is a woman enjoined to conceal her deed of mercy? Is a husband also the left hand of his wife? A statement most absurd. Or if any one thinks that they are left hands to each other; if any part of the family property be expended by the one party in such a way as to be contrary to the will of the other party, such a marriage will not be a Christian one; but whichever of them should choose to do alms according to the command of God, whomsoever he should find opposed, would inevitably be an enemy to the command of God, and therefore reckoned among unbelievers,— the command with respect to such parties being, that a believing husband should win his wife, and a believing wife her husband, by their good conversation and conduct; and therefore they ought not to conceal their good works from each other, by which they are to be mutually attracted, so that the one may be able to attract the other to communion in the Christian faith. Nor are thefts to be perpetrated in order that God may be rendered propitious. But if anything is to be concealed as long as the infirmity of the other party is unable to bear with equanimity what nevertheless is not done unjustly and unlawfully; yet, that the left hand is not meant in such a sense on the present occasion, readily appears from a consideration of the whole section, whereby it will at the same time be discovered what He calls the left hand.
8. “Take heed,” says He, “that you do not your righteousness before men, to be seen of them; otherwise you have no reward of your Father which is in heaven.” Here He has mentioned righteousness generally, then He follows it up in detail. For a deed which is done in the way of alms is a certain part of righteousness, and therefore He connects the two by saying, “Therefore, when you do your alms, do not sound a trumpet before you, as the hypocrites do in the synagogues and in the streets, that they may have glory of men.” In this there is a reference to what He says before, “Take heed that you do not your righteousness before men, to be seen of them.” But what follows, “Verily I say unto you, They have received their reward,” refers to that other statement which He has made above, “Otherwise you have no reward of your Father which is in heaven.” Then follows, “But when you do alms.” When He says, “But you,” what else does He mean but, Not in the same manner as they? What, then, does He bid me do? “But when you do alms,” says He, “let not your left hand know what your right hand does.” Hence those other parties so act, that their left hand knows what their right hand does. What, therefore, is blamed in them, this you are forbidden to do. But this is what is blamed in them, that they act in such a way as to seek the praises of men. And therefore the left hand seems to have no more suitable meaning than just this delight in praise. But the right hand means the intention of fulfilling the divine commands. When, therefore, with the consciousness of him who does alms is mixed up the desire of man's praise, the left hand becomes conscious of the work of the right hand: “Let not, therefore, your left hand know what your right hand does;” i.e. Let there not be mixed up in your consciousness the desire of man's praise, when in doing alms you are striving to fulfil a divine command.
Source: Our Lord's Sermon on the Mount (New Advent)