Using the eyeball test, that is, reading the Gospel of John and the Letters of John straight through, one can see an affinity between the Gospel and the Letters, though the theory of a separate community is not as strongly embraced today as it was half a century ago. The Biblical scholar Toan Do wrote the commentary on John’s Letters for The Paulist Biblical Commentary  and in his introduction Do writes this: “”A sound explanation is that the Johannine Epistles were composed separately [from the Gospel] and were circulated for both personal and communal reading among the churches.” [PBC, p. 1552] Do puts the date of composition of the Gospel around 95 AD and the letters between 100 and 110 AD. The consensus of scholarship places the composition of these letters in Asia Minor, specifically western Turkey.
Whether the Apostle John, or another apostle, or a scribe of John’s finished the Gospel, the Church has always considered the Gospel of Apostolic inspiration, and thus the Epistles as inspired commentaries of the theology of the Gospel. This body of literature places special emphasis upon themes vital to Church identity, the priority of love and the fact that Jesus is fully human as well as fully divine. The two great doctrines around which our tradition is built were coming to fruition and understanding in the Johannine era, i.e., the Incarnation and the Resurrection. Curiously, the harder of the two proved to be the Incarnation. There were wholesale defections or schisms which plagued the early Church, most of them dealing with the mistaken notion that Jesus was a “faux human,” a divinity who only appeared to be a man.
In Greek the verb for “to demonstrate” or “to show” is doceo, and eventually the heresy known today as Docetism argues in various ways that those who claimed to have seen Christ in his flesh were viewing a projection, so to speak, rather than a human like us in all things but sin, bound by the limits of space and time. Docetism in its raw form renders Jesus’ crucifixion useless and helpless in the forgiveness of sin on the grounds that the perfect sacrifice on Calvary never really happened.
The other heresy which plagued the infant Church along with Docetism was Gnosticism, from the Greek word for “knowledge.” Gnosticism was already in circulation as a philosophy before the birth of Christ, holding that material things are evil and only mystical spiritual realities were true. In its Christian iteration, only “sacred knowledge” shared with the chosen had the power to save. The idea of matter=evil percolated into Catholic thought long after the Johannine era, impacting St. Augustine’s teachings on sexuality four centuries after Christ.
As among the last writings of the New Testament in terms of date of composition, the letters of John squared off with the established enemies of the Apostolic Tradition, both from within and outside the Church. The author or authors of these letters thus labored with a two-front challenge: to defend and enforce the holiness of the Church while protecting it from errors of thought and practice. 1 John 1:1-4 bears a striking similarity to the beginning of John’s Gospel, emphasizing the visibility of the Word [God] multiple times and affirming fellowship among fellow believers.
In 1 John 1: 5-10, the author speaks of God as light; he equates true Christian faith with a conduct of walking always in light. Verses 6 and 7 speak of the importance of living in light, i.e., believing what the Church believes. To acknowledge one’s sins, in this context sins of disbelief, enables us to enjoy the saving fellowship of the church assembly, the body that has been cleansed by “the blood of his Son, Jesus….” This is a sweep at the Gnostic/Docetic element who spread division in the Church and denied the power of Christ’s blood to save. Many commentators believe that this letter, 1 John, was intended as a teaching statement for the general Christian Church, with the other two letters directed to specific communities.
Chapter 2 continues the theme of Christ’s blood [i.e., his full human sacrifice on the cross] as redemptive in saving us from sin and darkness. The author uses language similar to John’s Gospel: “The way we may be sure that we know him is to keep his commandments.” As Do explains, Chapter 2 emphasizes the closeness of Jesus to his followers, beginning the section with the invocation, “my children.” The closeness of the church is wounded, in the author’s mind, when even one member sins by deviating from the command of God to love one another. Unlike the Gnostics, the true Christian believer’s fidelity is a commitment to love his neighbor and seek the forgiveness won by Christ in his flesh. There is no “secret key” restricted to a few, and one will be judged by conduct in the here and now, in flesh and bones.
The author does use the term “world” with varied nuances. The saving forgiveness of the Father is extended to the whole world, but in Chapter 2 the believer is urged not to “join the world,” which stands in need of forgiveness. The Gospel and Letters share a feeling of the temporary nature of this world; in the Passion narrative of John’s Gospel, Jesus tells Pilate that “my kingdom is not of this world.” The author of the Gospel uses the parallel of day and night as a literary device to make this distinction. Judas leaves the Last Supper “at night” to betray Jesus. The Samaritan woman, by contrast, finds faith at high noon. These Letters carry something of this motif into their teaching.
The final three chapters of 1 John speak of a love ethic. Chapter 3 is sobering on this point, calling to mind the murder of Abel by Cain to show the violent excesses resulting where brother hates brother, instead of the other way around. Chapter 4 addresses “the testing of spirits,” an idiom for the ideas and practices of a Christian community. The author provides a rule of thumb in discerning thoughts and deeds: “every spirit that acknowledges Jesus Christ come into the flesh belongs to God.”
Chapter 5 repeats and reemphasizes much of what has been said before. We can guess that the author believed these basic themes needed repeating, having seen repeated disunity in the community and constant denial of the power of God to save his children through the blood of Christ. Christian communities at the end of the first century were for the most part small islands of believer struggling not just to preach to the strange worlds around them, but to clarify what it was that kept them united and courageous in the first place.