What is the eternal gospel? It is the gospel we preach: that Jesus Christ died for our sins and was raised from the dead on the third day according to the Scriptures, and that He appeared to faithful men and women who bore witness to this truth with their mouths, with their pens, and with their lives. It is the gospel of the Son of God, the gospel of the Messiah of Israel, and the gospel of the kingdom of God. This gospel was declared by Jesus Himself, and it had been preliminarily declared by his cousin John the Baptist.
The kingdom is eternal and it is here. Therefore, we shall always be declaring the kingdom of heaven on earth. Those who walk in truth and righteousness are those who see and seek the kingdom. The good news never ends.
The eternal gospel is that all men shall be saved and come to the knowledge of the truth. The eternal gospel is that God has fulfilled all prophecy in the coming of His kingdom. Not a jot or tittle remains unfulfilled. The eternal gospel is that Jesus Christ came again, just as He promised, just when He promised. The eternal gospel is that we may be saved from the judgment and wrath brought on by our sins by walking in faith and obedience to Jesus Christ.
The eternal gospel has nothing to do with regularly attending church, church programs, sacraments, church growth, or anything of the kind. These things obstruct access to the kingdom of God because they divert attention from the omnipresent Jesus, and there is no good news in them.