Caution against False Teachers
This and 3 John were Personal Little Notes to Friends whom John expected soon to visit. He wrote others letters (1 John 2:14; 3 John 9), possibly many of them. Personal Letters, such as these, on account of their brevity and private nature, would be less generally read in Christian Assemblies than Church Letters, and consequently be less widely known. These Two Little Epistles, under the guidance of God Spirit, were rescued from oblivion, and preserved for the Church, possibly, by being attached to a copy of 1 John in the particular Church or Churches where they had been received.
The Elder, 1
The other Apostles had all passed on, years before. John alone was left, the Primate of all Christendom, the last surviving Companion of Jesus. How appropriate the title.
The Elect Lady, 1
She may have been, literally, a Person, a well-known and prominent Woman, somewhere near Ephesus, in whose Home a Church met. Or, she may have been a Church symbolically called by a Woman's Name. Her Elect Sister (13), another prominent Christian Woman Leader, in the Congregation in which John was resident, or the Congregation itself.
The word "Truth" is used 5 times in the first 4 verses. Love in Truth (1). Know the Truth (1). Truth Dwells in us (2). Grace, Mercy, and Peace, in Truth (3). Walking in Truth (4). Truth is the True Doctrine of Christ (9), that He is the Son of God, and that Following Him means Walking in His Commandments (6), and His Chief Commandment is that We Love One Another (5).
False Teachers, 7-11
These already had been referred to in 1 John 2:18-29: going from Church to Church, Teaching, in the Name of Christ, Doctrines that were utterly subversive of the Christian Faith. This Letter seems to have been written to caution the Elect Lady to be on her guard, and refuse Hospitality to such Teachers. The warning is prefaced with an exhortation to Love (5, 6), as if to indicate that the Practice of Christian Love does not mean that we should give encouragement to Enemies of the Truth.