John explains his reason for writing this letter: “that you may have fellowship with us” (v. 3a). Fellowship (κοινωνία) refers to “close association involving mutual interests and sharing” (BDAG). We need to avoid thinking this fellowship is like modern Christian fellowship (snacks after church in the fellowship hall). The word can refer to sharing of resources, like a church koinonia fund. Romans sometimes joined voluntary associations which had some benefits, like a funeral association. You gave money to the group and they took care of your burial when you died. Local Christian churches had a similar purpose, people shared food and other resources with others so all were taken care of. Remember Paul giving instructions on care for widows, or the care for widows in Acts 6. But the word can also refer to a partnership, something like an organization that wants to “partner with you” (i.e., share your resources by getting you to give to their capital campaign).
John’s point here is if the reader wants to continue to have a fellowship relationship with the apostolic group he represents, then the reader will accept his testimony as true and respond properly to that testimony (believe right about Jesus and love one another, etc.) If the readers do not have fellowship with author then they have fellowship with the succecessionists.
The second reason for the letter is “so that you may have fellowship with the Father and Son” (v. 3b). On the one hand, this refers to eternal life, but in another sense “having fellowship” here means the believers can have a real sharing of resources with God himself. If one is in fellowship with God, then they also share fellowship with the Son of God, Jesus, the Messiah (3b). The phrase “son of God” might get overlooked here, but this is a messianic title (Psalm 2:6-7, based on the Davidic Covenant in 2 Sam 7:14, cf. Romans 1:4, Jesus is the “Son of God in power”).
It is important to keep the Jewish background in mind when we read Christ as well, this is a title, the Messiah. The purpose of John’s Gospel is that the reader would believe “Jesus is the Messiah, the Son of God.” This may be something the opponents deny (we will return to the reasons for this later). In 1 John 2:22 “Who is the liar but he who denies that Jesus is the Christ?”
Finally, John says he is writing this letter “so that our joy might be compete” (v. 4). There is a well-known textual variant here, some manuscripts have “your joy,” there is only one letter difference and the two words sound similar enough to account for the variant. Most modern translations have “our joy,” the KJV has “your joy.” Metzger favors the first person plural, “our joy” since it fits the context better. The writer will have complete joy when the readers believe in his testimony concerning the Word of Life (A Textual Commentary on the Greek New Testament, Second Edition, 639).
The purpose “completed joy” alludes to the Gospel of John. In John 3:29 John the Baptist refers to his role as a friend of the bridegroom (Jesus) as complete now that the bridegroom as arrived. He says “Therefore this joy of mine is now complete.” The Greek is very similar to 1 John 1:4. In John 15:11 Jesus says the things he has spoken things to his disciples so that their joy may be full and in 16:20-22 he says that after the resurrection their joy will be restored and no one can take their joy away from them. In John 17:13 Jesus prays that his disciples may have his joy fulfilled in themselves.
These three reasons for writing the short letter now known as 1 John all relate to how the writer understands those who are part of the apostolic community and those who are outside of that community. John will further define what it means to continue as part of his community. Some of these will right teaching about Jesus, others will be proper behavior as a Christian, summarized in the phrase “love one another.”