As in his Gospel and the First Epistle, John does not give us his name. Here he calls himself "The elder"which could refer to a church office (as in Acts 14:23, 15:6, 1 Titus 1:5), but an older man seems more likely (as in 1 Timothy 5:1, 1 Peter 5:1). That he is the same person as the writer of the First Epistle is clear from ten identical expressions used in that Epistle and this one:

1) know the truth (1 John 2:21, 3:19)
2) abides in (1 John 2:6, 27, 3:6, 4:15)
3) walking in (1 John 1:6, 2:11)
4) new commandment (1 John 2:7-8)
5) love one another (1 John 3:11, 23)
6) as you have heard it from the beginning (1 John 3:11)
7) antichrist (1 John 2:18, 22)
8) the Messiah has come in the flesh (1 John 4:2)
9) the Father and the Son (1 John 2:23)
10) that our joy may be complete (1 John 1:4)

It seems likely that the term "elect lady" is metaphorical of a church congregation (see comment on verses 1, 4, 5, 10, 13).
2 John therefore looks like the covering letter for the First Epistle. And that Epistle would then have been copied by the receiving church and sent out to other churches.

1 Some think this letter was addressed to a mother (perhaps a widow) and her children. If this was a literal woman, then she had a sister in Ephesus or wherever John is writing (13). But it makes much more sense to assume that in both verses (1 & 13) the "elect lady" is metaphorical of a church and its members. Paul does use the same term 'elect' (eklektos meaning 'chosen') for a man named Rufus (Romans 16:13). But then churches are also chosen (Ephesians 1:4, 1 Peter 1:2). And since John tells us that all who have known the truth also love this 'elect lady', the metaphorical title of a church seems certain (as in verse 5). The name 'elect lady' may have been used to avoid identification in case the messenger was intercepted by the Roman authorities (see verse 12).

2 The good news as the truth of God is very important for John. In the Gospel he had recorded Jesus' words, "I am the way, and the truth, and the life" (John 14:12). And all three of these words are picked up in the First Epistle (1 John 1:7, 2:21-22, 3:14-15). John also heard Jesus' prayer when he said of the apostles that they now "know in truth that I came from you and they have believed that you sent me" (John 17:8). The opposite of God's truth is the lie (1 John 2:22, 27, 3:19). And the lie is the antichristian denial (1 John 2:22-23, 2 John 7) that Jesus was the eternal Son of God come in the flesh. Without that essential component of faith Christianity would be nothing more than the moralizing of Jesus as a good man.

3 The emphasis on both the Father and the Son was a key thread in John's Gospel (John 1:1, 14, 18, 3:16, 5:17-23, 26-27, 36-38, 6:37-40, 46, 8:54-58, 10:17-18, 33-38, 12:49-50, 14:6, 17:1, 8 )

4 If John was writing to a literal mother, we would have to assume that he has met some of her physical children in the place he is writing from. It is more likely these are some traveling members of the church whom he has met on his journeys. Again John is emphasizing the truth (see verse 2).

The Greek could be translated "according to the doctrine (1 Timothy 6:14) which we received from the Father."

5 The commandment that we "love one another" (see 1 John 3:11, 23) would hardly be appropriate if addressed to a particular woman. The old commandment is what John had written in the first twelve chapters of the Gospel. The new commandment was given at the last supper (John 13:34, 15:12), but for the readers of John's Epistle it was already well known.

6 Love is walking with the Son of God, and obeying his new commandment to love other members of his adopted family. It is interesting that Adam and Eve did not want to walk with the Son of God (Genesis 3:8), but Enoch, Noah and Abraham are described as walking with him (Genesis 5:24, 6:9, 12:1,4,13:17, 17:1).

7 Jesus had warned his disciples that deceivers would precede his coming to destroy the temple (Matthew 24:5, 23-24), and John has concluded that "last hour" (1 John 2:18) was now at hand (1 John 4:1-3). The antichristian deception involved the denial that Jesus was the eternal Son of God, who had already come in the flesh (as in 1 John 4:2).

8 Being taken in by this denial (which is common among scholars in our day) is disastrous for Christian faith. Paul distinguishes between the foundation which is the Messiah, and what we build on that (gold, silver, wood, hay, straw), and comments that "if what has been built on the foundation, the builder will receive a reward" but if our work is burned up, we suffer loss
(1 Corinthians 3:12-15)

9 The Greek text says "The person who takes a lead without remaining in the teaching of the Messiah, does not hold on (have a grip on) God." As in the next verse, this refers to people who try to introduce false doctrine. We must distinguish the experience of a heart looking to God (which may still lack a correct understanding) from the theological explanation which Jesus gave about his relationship to the Father (see the references given in verse 3). What false teaching does is to confuse us and take away our assurance. A proper understanding of Jesus as the eternal Son of God (verse 7) is what good theology imparts. Without it we do not have God's truth.

10 The coming to you is in the plural, so this refers to a congregation receiving false teachers to teach among them. As a school of the Holy Spirit, the church welcomes any who want to learn (Romans 15:7). The one kind of person such a school cannot welcome is someone who wants to come in and teach the exact opposite of what the school is about. The danger of this is so serious that we must ruthlessly keep out such 'antichristian' preachers (1 John 2:18, 22-24, 4:1).

11 Peter finally confessed "You are the Messiah, the Son of the living God" and the church is built on this foundation (Matthew 16:16-18). By welcoming someone who wants to teach that Jesus is not the Messiah the Son of God we ally ourselves with those who crucified him.

12 Writing on paper was dangerous. Better the joy of talking "face to face".

13 As we have suggested above 'the children of your elect sister' are not the children of the lady's sister, but the members of the church from where John is writing