5:13-16 Ye are the salt of the earth. Mankind, lying in ignorance and wickedness, were as a vast heap, ready to putrify; but Christ sent forth his disciples, by their lives and doctrines to season it with knowledge and grace. If they are not such as they should be, they are as salt that has lost its savour. If a man can take up the profession of Christ, and yet remain graceless, no other doctrine, no other means, can make him profitable. Our light must shine, by doing such good works as men may see. What is between God and our souls, must be kept to ourselves; but that which is of itself open to the sight of men, we must study to make suitable to our profession, and praiseworthy. We must aim at the glory of God.

14. Ye are the light of the world—This being the distinctive title which our Lord appropriates to Himself (Joh 8:12; 9:5; and see Joh 1:4, 9; 3:19; 12:35, 36)—a title expressly said to be unsuitable even to the highest of all the prophets (Joh 1:8)—it must be applied here by our Lord to His disciples only as they shine with His light upon the world, in virtue of His Spirit dwelling in them, and the same mind being in them which was also in Christ Jesus. Nor are Christians anywhere else so called. Nay, as if to avoid the august title which the Master has appropriated to Himself, Christians are said to "shine"—not as "lights," as our translators render it, but—"as luminaries in the world" (Php 2:15); and the Baptist is said to have been "the burning and shining"—not "light," as in our translation, but "lamp" of his day (Joh 5:35). Let it be observed, too, that while the two figures of salt and sunlight both express the same function of Christians—their blessed influence on their fellow men—they each set this forth under a different aspect. Salt operates internally, in the mass with which it comes in contact; the sunlight operates externally, irradiating all that it reaches. Hence Christians are warily styled "the salt of the earth"—with reference to the masses of mankind with whom they are expected to mix; but "the light of the world"—with reference to the vast and variegated surface which feels its fructifying and gladdening radiance. The same distinction is observable in the second pair of those seven parables which our Lord spoke from the Galilean Lake—that of the "mustard seed," which grew to be a great overshadowing tree, answering to the sunlight which invests the world, and that of the "leaven," which a woman took and, like the salt, hid in three measures of meal, till the whole was leavened (Mt 13:31-33).

A city that is set on an hill cannot be hid—nor can it be supposed to have been so built except to be seen by many eyes.