Matthew 5:14

Ye are the light of the world--This being the distinctive title which our Lord appropriates to Himself (Joh 8:12; Joh 9:5; and see Joh 1:4, Joh 1:9; Joh 3:19; Joh 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" (Phi 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.

— Jamieson-Fausset-Brown Commentary (public domain)