The Law kept by Sincerity.
I. It was the custom for great personages princes and governors and such like when making high procession through some favoured province, to sound a trumpet before them, and scatter largess of gold and silver, whereby they gained the good will of the poor. Our Lord likens the almsgiving of the Pharisees to this kind of lordly display of munificence. Their alms were never distributed without their taking good care, one way or other, to let the good deed be known, so that they might get honour among men.
II. Note that the guilt of this conduct lay entirely in the spirit which actuated them. Jesus detected that spirit. It was not the publicity of their conduct in itself which He blamed, but the ungodly motive which led to that publicity; and I think it is necessary to bear that in mind, lest we may get in the way of judging others, and judging them unjustly, by the mere external appearance. The really compassionate and liberal man is often put into the front, and obtains a prominence from which he would otherwise gladly shrink; and he gets this position, not with the view of exalting him, but in order that his example may stimulate and encourage others. The difficulty is to reconcile these two things: to avoid all ostentation, and yet at the same time to get all the advantage of generous Christian example.
III. The phrase, "Let not the left hand know what the right hand doeth," is a proverbial expression, implying that our charity is not to be done ostentatiously so as to be seen of men, nor yet self-righteously so that we may pride ourselves upon it. That almsgiving is, and always will be, a duty is plainly involved here. The charity which does not let its left hand know what its right hand doeth is manifestly a spirit of meekness and simplicity, which neither courts the observation of others, nor cares to dwell on its own excellence, but drops its beneficence like dews, in the silence and darkness, so that its presence is known only by the blessing which it leaves behind. But the man who gives an alms, and then settles down in the pride and contentment of his own deed, hath therein his reward. He has taken all the beauty from his work. It has lost its Divine character as a deed of true pity, and become an act of merest vanity.
W. C. Smith, The Sermon on the Mount,p. 162.
References: Matthew 6:1-6; Matthew 6:16-28. E. Bersier, Sermons,2nd series, p. 35.Matthew 6:1-8. Homiletic Quarterly,vol. v., p. 521.