John is addressing gnosticism; addressing the question of whether Jesus is really God.
Praying some kind of "magic" sinner's prayer is not what saves. It is faith in Jesus that saves.
In the gnostic way of thinking, sin didn't matter.
1 John 1:8-10 (MSG) If we claim that we’re free of sin, we’re only fooling ourselves. A claim like that is errant nonsense. On the other hand, if we admit our sins—make a clean breast of them—he won’t let us down; he’ll be true to himself. He’ll forgive our sins and purge us of all wrongdoing. If we claim that we’ve never sinned, we out-and-out contradict God—make a liar out of him. A claim like that only shows off our ignorance of God.
John was there from the beginning and saw it all -- all of Jesus' ministry. (1 Jn 1:1-2)
Eyewitness testimony is very powerful.
(1 Jn 1:3-4) You can experience it too. We want you to enjoy this, too. Your joy will double our joy!
Think about someone you know that does not know and is not following Jesus. If they were to place their faith in Jesus, would it not increase your own joy -- greatly?
The Christian life is not about avoiding damnation, but about relationship with God. The former has us looking inward. The latter has us looking outward -- toward God, toward other believers, toward the lost.
A sense of amazement is missing in the modern church. How can a true faith in Jesus not radically change the way we live our lives? Why is it OK to wear a cross around our neck if nothing else in our lives changes?
Is communion with God amazing to you? Do you have a sense of amazement?
Is your motive for telling others about God to get them on "your side" that they validate your belief system?
John enjoys God and cares enough about others that he wants them to experience that joy too.