Why Was the Epistle of John Written?

 
"These things have I written unto you that believe on the Name of the Son of God; that ye may KNOW that ye Have Eternal Life, and that ye may believe on the name of the Son of God." (1 John 5:1313These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God. (1 John 5:13).)