The illustration of the text has a twofold reference. It bears on what went before, and also on what follows. If we lay up treasures on earth, that will produce an evil eye; if we try to serve God and mammon, that will destroy the single eye. Look at the passage in both these aspects.
I. If the light within you be true, if it be your real heart's desire to see what is right, if your affections are set on those things which are just and pure and lovely, the things heavenly and eternal, then shall your eye be single, and as ye look forth on the world ye shall be able to estimate its treasures at their proper value, for they will have lost to you the glamour and the fascination which they exercise over others. Their inherent emptiness, their essential vanity, their utter precariousness, their certain brevity will be all naked and open to the clear vision of faith, which sees them in their true character and values them at their proper worth.
II. Consider next the evil eye, as it is produced by the effort to serve both God and mammon. The influence of utter and unmitigated worldliness, when a man gives himself to it heartily and without scruple or drawback; that is, as we have seen, to blind his mind altogether to the higher concerns of the spiritual world. Therefore he never troubles himself about them; can see no need of them, and no value in them. That is a sad state of darkness; but it is a sort of honest darkness, and is consistent with a certain genuineness of character. But the effort to serve both God and mammon produces a kind of self-deception, which is to my mind greatly more pernicious and worse to overcome than the former. The thorough worldling knows himself to be so, and his evil eye sees nothing else worth troubling himself about. The other, however, fondly persuades himself that he is not a worldling, that he is, indeed, far superior to the worldling; his evil eye sees, in a measure, what is right and good, but only regards it so far as may be necessary to keep his mind easy in its worldliness. Thus the light which is in him serves more effectually the purpose of darkness.
W. C. Smith, The Sermon on the Mount,p. 224.
Reference: Matthew 6:22; Matthew 6:23. Preacher's Monthly,vol. vii., p. 378.