Let not thy left hand know By this expression he means, that we ought to be satisfied with having God for our only witness, and to be so earnestly desirous to obey him, that we shall not be carried away by any vanity. It frequently happens, that men sacrifice to themselves rather than to God. Christ therefore wishes, that we should not be distracted by indirect thoughts, but go straight to this object, that we may serve God with a pure conscience.

Let not thy left hand know - This is a proverbial expression, signifying that the action should be done as secretly as possible. The Hebrews often attribute actions to members which properly belong to persons. The encouragement for performing our acts of charity in secret is that it will be pleasing to God; that he will see the act, however secret it may be, and will openly reward it. If the reward is not granted in this life, it will be in the life to come. In multitudes of cases, however, alms given to the poor are "lent to the Lord" Proverbs 19:17, and will be repaid in this life. Rarely, perhaps never, has it been found that the man who is liberal to the poor has ever suffered by it in his worldly circumstances.

Let not thy left hand know - In many cases, works of charity must be hidden from even our nearest relatives, who, if they knew, would hinder us from doing what God has given us power and inclination to perform. We must go even farther; and conceal them as far as is possible from ourselves, by not thinking of them, or eyeing them with complacency. They are given to God, and should be hidden in Him.

But when thou dost alms,.... Do it so privately, and with so much secrecy, that, if it was possible, thou mightest not know it thyself, much less make it known to others:
let not thy left hand know what thy right hand doth; acquaint not thy nearest and dearest friend with it; let not one that sits at thy left hand know what thou art doing with thy right hand; it is a proverbial and hyperbolical phrase, expressing the secrecy of the action. It is a Jewish canon (p), that
"he that gives a gift to his friend out of love, may make it known, , "but not if it be by way of alms".''
(p) Piske Tosephot in Sabbat. c. 1. art. 134.

But when thou doest alms, let not thy left hand know what thy right hand doeth--So far from making a display of it, dwell not on it even in thine own thoughts, lest it minister to spiritual pride.

Let not thy left hand know what thy right hand doeth. A strong expression, to indicate that there must be no publishing of our alms deeds.
That thine alms may be in secret. It is not concealment that is required, so much as to avoid ostentation.
Openly. Literally, "in the open place," in the last day, when every secret thing is made manifest.

Let not thy left hand know what thy right hand doth - A proverbial expression for doing a thing secretly. Do it as secretly as is consistent, With the doing it at all. With the doing it in the most effectual manner.