I, II and III John were written by John, a close associate and disciple of Jesus.
I John opens with his heartfelt testimony and his purpose for sharing , 1:2 "The life appeared; and we proclaim to you what we have seen and heard, so that you also may have fellowship with us. And our fellowship is with the Father and with his Son, Jesus Christ. We write this to make our joy complete."
He encourages brotherly love, 2:10 "Whoever loves his brother lives in the light." In 3:18 John urges Christians to put love into action "Dear children, let us not love with words or tongue but with actions and in truth."
In 3:23 John sums up the simplistic beauty of Christianity, "And this is his command, to believe in the name of his Son, Jesus Christ, and to love one another as he commanded us."
He also refers to the relationship that is fostered when Christians obey 3:24, "those who obey his commands live in him and he in them. And this is how we know that he live in us: We know it by the Spirit he gave us."
He mentions the notion that God is love, 4:16 "And so we know and rely on the love God has for us. God is love. Whoever lives in love lives in God and God in him. In this way love is made complete among us..."