Anyone who believes in the Son of God has this testimony in his heart. Anyone who does not believe God has made him out to be a liar, because he has not believed the testimony God has given about his Son. God has given Christians the most precious gift possible and the responsibility to tell others how they, too, can receive that gift. Entrance into the kingdom of God is salvation. Receiving eternal life is the same thing as receiving salvation, because salvation is being saved from sin, which brings death.
Eternal life requires forgiveness of sin.
Forgiveness of sin, or justification, comes only by faith in Jesus Christ. Jesus is both Judge and Redeemer. And he is also King of that kingdom. The gospel of the kingdom of God is the gospel of salvation in Jesus Christ. Let's go further.