v1: John wrote to the elect lady and her children. The exact meaning here is not known, but it is likely to refer to a church, and v13 to another local church. This is a personal letter to give encouragement, and to bring a warning.
v2: Knowing the truth and loving the brethren; John's themes continue from the first letter; their content is essential and consistent.
We avoid love without truth; which is sentimentalism, and ecumenism, and is not really love.
We avoid truth without love; which is harsh and critical, and is not really truth.
Love and truth characterised Jesus Christ. The world struggles to reconcile truth and love because it understands neither. The spiritual man has understanding in Christ, and can pursue both love and truth.
There is truth for ever, in Heaven, where there is no lie, no half-truth, no deceit.
v3: The opening greeting, praying grace and peace on his readers, is varied with a reference "in truth and love," reflecting John's concern.
v4: John's joy, seeing God's command fulfilled, in younger Christians walking in the truth.
v5-6: No new commandment, as in 1 John 2.7; 3.11, but that we love others, as God has commanded.
v7-11: The burden of the letter is the twin issues of discernment and vigilance, as in 1 John 4.1, since there are many false teachers.
v7: The test of deceivers is that they do not confess that Jesus Christ has come in the flesh, 1 John 4.2-3. They are described as deceivers, even antichrist, and there are many of them.
v8: Look to yourselves, 1 John 5.18, personal vigilance, ensuring a close walk with the Lord. If we are taken in by these deceivers, then we will lose the fullness of Christ's blessings.
v9: The "teaching of Christ", consistent with "the faith" of Paul's letters; true doctrine centres upon Jesus Christ.
John uses the phrase "anyone who runs ahead" (NIV, RAV margin) to describe those who transgress by not following the teaching of Christ. The idea is of someone pursuing their own agenda, rather than submitting to God's truth.
v10-11: The command is not to even greet the false teachers, for that is to share in their evil deeds.
v12-13: John's intention was to meet and speak face to face, preferring that means of communication.