The Author is John the Apostle.
This book was written to dispel doubts and to build assurance by presenting a clear picture of Jesus Christ. Entering into human history through the incarnation, the Son of God became the very embodiment of God in the flesh – seen, heard, and touched by the author, John the Apostle.
John walked and talked with Jesus Christ, saw him heal, heard him teach, watch him die, met him arisen, and saw him ascend into heaven. John knew God – he had lived with him and had seen him work. As the elder statesman in the church, he wrote this book.
In it he presents God as light, as love, and as life. He explains in simple and practical terms what it means to have fellowship with God. At the same time, false teachers had entered the church, denying the incarnation of Jesus Christ. John wrote to correct their serious errors and for believers to combat modern heresies.
First John was written to help believers know the reality of God in their lives, to assure you that you have eternal life through Jesus Christ, and to encourage you to have continual fellowship with the God who is light and love.