While first John does not name its author, very early in  Church history the apostolic fathers, including Polycarp—who studied under John, assigned this epistle to John the Apostle.  Ignatius of Antioch also claimed John authored this letter. Interestingly, Irenaeus wrote that John resided in Ephesus. This is important  since the Book of Revelation addresses the churches of Asia. Hence, it is quite probable that the three epistles of John are addressing the same believers in Ephesus.   

Moreover, we know that the “beloved disciple” implies John. Equally important is the fact that the author claims to have been an eyewitness of Christ (1:1-4), including claiming to have “touched” him—the “Word of Life”. Thus, to assign this epistle to the second century  ignores the eyewitness testimony of the writers of the Gospels.   In addition (and equally as important), this epistle was quoted widely in the second century, which implies that it was written long before. In comparing the style and vocabulary in 1, 2, 3, John with the Gospel of John, we observe striking similarities. Thus, if we establish that John penned his gospel, then based on the literary evidence, we can be reasonably certain that the epistles  are his as well.   

As for the dating of this epistle,  we know that early second century believers quoted from it. Thus, to give it a first century date is reasonable. Furthermore, the  strand of Gnosticism that John responded to is much more developed than what we observe in the epistles, for example, of Peter and Paul. These apostles refuted a primitive and embryonic kind of Gnosticism, while John was refuting a more established and intellectually rigorous heresy. Given that John died before the close of the first century and the fact that Gnosticism flourished in the mid-second century and onward, we date this letter in the last part of the first century around A.D. 96. While some date this book before the destruction of Jerusalem in A.D. 70, such an early date does not work because the heretical ideas that John is countering would not have had enough time to  develop into  the more mature synthesis of Greek dualism and Eastern mysticism.   

We mentioned earlier that prior to his  death John lived and ministered to the saints in Ephesus. In addition to this epistle having the tone of an older man by its frequent habit of referring to its recipients as “little children”, we can assign this letter a later date on other grounds. For example, Paul visited Ephesus multiple times between A.D. 53 and 56. Then, in A.D. 63, Timothy visited Ephesus with Paul as well.  In fact, Timothy was still in Ephesus when Paul wrote him some three or four years later (ca. A.D. 67). Yet, we have no sources indicating that Timothy and?.  ? John, having recognized the threat of Gnosticism and its infiltration by false teachers, sought to set the record straight. John appears to be writing without collaboration against a more virulent Gnostic form.  As  we read in Genesis,  man was created in God’s image, and along with all matter was declared “very good.”  The Gnostics turned this idea on its head, and claimed that all created “matter,” including man’s flesh, was evil, or at least created at the lowest level of existence.  John deemed this false dualism between man and the created order and God to be heretical. We see Paul in his first epistle to the church at Corinth addressing a primitive yet similar version of Gnosticism  that possessed a very low view of matter or the flesh. Thus, it is no wonder that we see a stronger and more fully developed  Christology in his first epistle. John's message in 1 John is fourfold: he wanted the joy of his readers to be "complete" (1:4), he admonished them to avoid sin but also to ask forgiveness if they did sin (1:9, 2:1), he wanted them to be able to refute false teachers (2:26), and he wanted them to know or be assured that they "have eternal life" (5:13).   

John  fought against the Gnostic heresy on two fronts in 1 John by emphasizing the two natures of Jesus Christ – his true humanity and his true deity (centuries later this theology of the union of the two natures would be called the “hypostatic union”). He says he heard, saw and even touched Jesus, confirming the actuality of His humanity. He pulled no punches, saying "Every spirit that confesses that Jesus Christ has come in the flesh is of God, ?and every spirit that does not confess ??that Jesus ?Christ has come in the flesh is not of God" (4:2-3). In fact, he said that those who deny the humanity of Jesus is "the spirit of the Antichrist" (4:3). He also confirms the deity of Jesus, calling Him "the true God and eternal life" (5:20), and affirming Him as one of the  Three in the divine unity (5:7).  

Finally, in addition to refuting Gnosticism, John also assures the believer of eternal assurance of salvation in Christ. While some maintain that John is stating the opposite, in reading 1 John in its plain reading, and by following the historical-grammatical interpretive method, the message of the assurance of one’s salvation is clear.