“And these things write we unto you, that your joy may be full” (I John 1:4).
In John’s first epistle, the apostle lists four basic reasons for writing this personal letter. Due to the fact that Christians have the same needs now as then, John’s instruction is right up-to-date.
The first reason why he wrote is found in the conclusion of I John 1:4: “And these things write we unto you, that your joy may be full.” There is a wonderful joy that comes to believers through fellowship with one another and the Lord, as is expressed in 1:3: “That ye also may have fellowship with us: and truly our fellowship is with the Father, and with his Son Jesus Christ.” Fellowship brought with it fullness of joy.
The second reason for writing is in 2:1: “My little children, these things write I unto you, that ye sin not.” Good advice! Sin brings nothing but misery and judgment. Sin is still “a reproach to any people” (Proverbs 14:34).
The third reason is found in I John 2:26: “These things have I written unto you concerning them that seduce you.” False teachers were trying to lead them astray, and John exhorts them to continue (v.24) in the good, sound doctrine that was the foundation of their faith from the beginning of their salvation.
The fourth reason is in 5:13: “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.” The believer can know—that is, have positive knowledge, beyond a shadow of a doubt, that he has eternal life. Salvation is a person: Christ, therefore, “He that hath the Son hath life” (eternal); “and he that hath not the Son of God hath not life” (v.12).
The hymn writer expressed it well: “Blessed assurance, Jesus is mine!” Or, as another beautiful hymn testifies: “More secure is no one, ever, than the loved ones of the Savior!” NPS