The book of 1 John is a series of General Letters to the believers of Christ. The purpose of the letters were to combat false teachings.
In Chapters 1-2: John reassures believers explaining, “…God is light, and in Him is no darkness at all” (1 John 1:5). He explained that if sins were confessed, those sinners would be made whole.
In chapters 3-5: John teaches about the love of God and that through His love He sent Jesus, to Atone for the sins of the world. John urges Christians to live by faith. Those who love God, will keep His commandments and those who keep His commandments will overcome the world!