John was one of the first men to follow Jesus. He knew Jesus well. But John also understood that Jesus is God (1 John 1:1-3).
John wrote this letter to all Christians. John wanted Christians to be certain that God loves them (1 John 3:1). Because God loves us, we too must love other Christians (1 John 3:17; 1 John 4:19-21).
This has always been the law of God (1 John 2:8). We must love other people.
John also wanted to remind Christians about some important facts:
Christians are God's children and they can enjoy friendship with him. (1 John 3:1 and 2:3)
They do not need to do wrong things. But God will forgive them if they confess their evil deed to God (1 John 2:1).
The Holy Spirit is with them to warn them about false ideas. (1 John 2:18-27)
They can be joyful and have true friendship with other Christians. (1 John 1:4 and 7)