"...
he had the eternal gospel to proclaim to those who live on the earth—to every nation, tribe, language and people" (Rev 14:6).
The gospel. Like a multifaceted jewel, Scripture describes the gospel with various terms, each looking at it from a different viewpoint: the gospel of God (Mk 1:14; Rom 1:1), the gospel of the kingdom (Mt 4:23; Mt 24:14), the gospel of the grace of God (Acts 20:24), the gospel of the glory of Christ (2 Cor 4:4), the gospel of salvation (Eph 1:13). In Revelation, it is the eternal gospel (Rev 14:6).
Why is the gospel eternal? The gospel is "eternal" because it points to a message that is permanently valid. It also provides the means to eternal life.