The three-word gospel  regularly reiterated by John the Baptist and Jesus:  Repent and Believe. John the Apostle puts some flesh on the bare bones in the first chapter of his letter named, appropriately, 1 John. 

First, what are we to Believe?  In verses 1-4, John makes the case that the historical, real-life, we saw him, touched him and listened to him Jesus is the Son of the eternal Father God!   Believe in HIM. Realize His majesty and Glory; worship Him as God; acknowledge Him as Lord and Savior just as the apostles and first believers have done.  Join the fellowship of believers.

Second, what are we to Repent about?  Verses 5-10 emphasize that God is light and can tolerate no darkness, sin, or unrighteousness.  You must acknowledge that you have sinned – “If we say we have no sin, we deceive ourselves, and the truth is not in us.”  Even worse, if we way we have not sinned, we are calling the One True God of the Universe a liar (verse 10)! 

But the Good News about the nature of sin is contained in verse 9:  “If we confess our sins, he is faithful and just to forgive our sins and to cleanse us from all unrighteousness.”  Confession leads to forgiveness and cleansing, which leads not only to fellowship with other believers but with God the Father and God the Son.

So, Repent and Believe is a call to confess sin, receive forgiveness, and join in the fellowship offered by the risen savior Jesus Christ.