In 1:6, John says that if we claim to have fellowship with Christ (i.e. that we believe and are children of God) but go on walking in darkness (i.e. living a life of sin like those who are in the darkness of unbelief), then we are lying. In 1:8 and 10, John adds that a believer who says that he no longer sins in any way is deceiving himself. In 2:1-2 John adds that a believer who sins in weakness of faith is fully forgiven by Jesus' saving work. But if a believer says, "I might as well just go on sinning because all my sins are forgiven anyway," that person is not one of God's children (2:4).

Then, in the rest of the letter John keeps reminding us to serve Jesus with our lives out of thanks to him for saving us. In this way what John writes is the same as what Paul wrote in chapter 6 of his letter to the Romans. In this chapter Paul repeats again and again that a believer can't use the full and free forgiveness that he/she has as an excuse to live in sin. Rather this gracious gift leads a believer to serve God with his/her life (Romans 6:1,6,11-12,15,19).

That is also John's message in 1 John 3:8ff. For someone to live a continual life of sin because his sins are all forgiven is contrary to the attitude of one who is a believer. A believer will fall into sin in his weakness of faith, but in sorrow he will repent of that sin and ask God's help to live a holy life. Full and free forgiveness does not lead a believer to just go on living in sin, but rather it leads him/her to live a life that shows his thanks for God's gracious gift of life and salvation.