Revelation 14:6

In this part of the chapter we have three angels or messengers sent from heaven to give notice of the fall of Babylon, and of those things that were antecedent and consequent to that great event. I. The first angel was sent on an errand antecedent to it, and that was to preach the everlasting gospel, Rev 14:6, Rev 14:7. Observe, 1. The gospel is an everlasting gospel; it is so in its nature, and it will be so in its consequences. Though all flesh be grass, the word of the Lord endureth for ever. 2. It is a work fit for an angel to preach this everlasting gospel; such is the dignity, and such is the difficulty of that work! And yet we have this treasure in earthen vessels. 3. The everlasting gospel is of great concern to all the world; and, as it is the concern of all, it is very much to be desired that it should be made known to all, even to every nation, and kindred, and tongue, and people. 4. The gospel is the great means whereby men are brought to fear God, and to give glory to him. Natural religion is not sufficient to keep up the fear of God, nor to secure to him glory from men; it is the gospel that revives the fear of God, and retrieves his glory in the world. 5.

— Matthew Henry's Concise Commentary (public domain)

Here begins the portion relating to the Gentile world, as the former portion related to Israel. Before the end the Gospel is to be preached for a WITNESS unto all nations: not that all nations shall be converted, but all nations shall have had the opportunity given them of deciding whether they will be for, or against, Christ. Those thus preached to are "they that dwell (so A, Coptic, and Syriac read. But B, C, ORIGEN, Vulgate, CYPRIAN, 312, read, 'SIT,' compare Mat 4:16; Luk 1:79, having their settled home) on the earth," being of earth earthy: this last season of grace is given them, if yet they may repent, before "judgment" (Rev 14:7) descends: if not, they will be left without excuse, as the world which resisted the preaching of Noah in the the hundred twenty years "while the long-suffering of God waited." "So also the prophets gave the people a last opportunity of repentance before the Babylonian destruction of Jerusalem, and our Lord and His apostles before the Roman destruction of the holy city" [AUBERLEN]. The Greek for "unto" (epi, in A and C) means literally, "upon," or "over," or "in respect to" (Mar 9:12; Heb 7:13). So also "TO every nation" (Greek, "epi," in A, B, C, Vulgate, Syriac, ORIGEN, ANDREAS, CYPRIAN, and PRIMASIUS). This, perhaps, implies that the Gospel, though diffused over the globe, shall not come savingly unto any save the elect.

— Jamieson-Fausset-Brown Commentary (public domain)