The following is found also in the "author/date" section of 2 & 3 John.
There is nothing within 2 or 3 John that points unambiguously towards a specific author. Irenaeus of Lyons in the early 2nd Century links 2 John with 1 John, and ascribes them both to John the Apostle and author of the fourth gospel. It appears that Origen was the first to mention all three, though according to Eusebius, he mentioned the second and third whilst pointing out that not all viewed them as authentic (Eusebius, op. cit., vi, 25).
Arguments that 2 and 3 John came from a different author than that of 1 John become evidently problematic when considering content and vocabulary, as several distinct thematic links exist between the first epistle and the last two. Much of the foundation of the view of separate authorship comes from the self-identification of the writer of 2 and 3 John within the text as ‘the elder.’
Many scholars (including Eusebius) view it as unlikely that John the Apostle would refer to himself as ‘the elder’, yet it is not impossible. In many ways, it would make sense, as the Apostle John was the last apostle left alive and reputedly lived to an old age. It would then be quite natural to refer to himself as ‘the elder’ as he truly was in a special position. If, on the other hand he had referred to himself as ‘the Apostle’, simply because he was the last apostle alive, this would have been "both presumptuous and out of character".
1 John is unanimously agreed by the early Church to have been written by John son of Zebedee (the Apostle), and it seems that the obvious similarities in content of 2 & 3 John with 1 John point towards his authorship of these as well.
Probably written during the early nineties - though it is impossible to be sure.
By the time of writing, many of the recipients of the letter would have been second or third generation believers - quite a different situation to the epistles of Paul, written thirty to forty years previous. By the content of the epistle, it seems that various forms of heresy had crept into the church, and the believers in Asia needed to be set straight on the true and apostolic faith in Christ.
Part of this heresy would likely have been that put forward by the Gnostics. Proponents of Gnosticism held that in essence, spirit is pure and matter is evil. Therefore the Gnostic despises the world and all in it, and sought release for their spirit through secret rituals and rites. Due to this view of all physical things as evil, Gnosticism naturally denied the Incarnation; for at its heart it teaches that the perfect God who existed solely in (perfect) spirit became physical; and therefore, according to the Gnostics, evil. Some (Docetists) believed that Jesus didn't come in flesh, but only appeared to have a body.
The result of these (and similar) teachings, was first the problem that according to both Gnosticism and the more extreme Docetists, Jesus could not have died on the cross - the central tenet of the Christian faith; and as it happens, an event that John was personally witness to. Second, believing evil to be solely associated with flesh allowed proponents of this view to justify sin in their lives by claiming it as the result of their flesh, not of their 'spirit'; and therefore once their spirit was freed from their flesh they would be free from sin.
In response to these views, John made clear the truth of the Jesus' identity, writing first of the many witnesses to his Incarnation (1:1-3; 4:2) and second that the Holy Spirit confirms this truth in the hearts of believers (2:20, 27; 3:24). Further, he warned that "No one who denies the Son has the Father" (2:23) whilst promising that "Whoever confesses that Jesus is the Son of God, God abides in him , and he in God" (4:15 cf 1:1-3,4:14; 2 John 9). Scattered throughout the book are also numerous warnings to beware of false teachers and teaching (e.g., 4:1-6; 5:21).
Quite obviously John desires to steer his readers towards truth as well as away from heresy, and so repeatedly advocates obedience to God's Commandment - to love one another. Far beyond the legalistic quantifications and interpretations evidenced by the Pharisees of Jesus' day, John sums up the law (as Jesus had done previously) by stating that love for God will show itself in love for one another (e.g., 2:7-11; 3:11-24; 4:7-21).
The overall message seems pretty clearly to be found in two points:
Remember what our Lord Jesus Christ said: "'Love the Lord your God with all your heart and with all your soul and with all your mind.' This is the first and greatest commandment. And the second is like it: 'Love your neighbor as yourself.' All the Law and the Prophets hang on these two commandments." (Matthew 22:37-40)