Greetings! This is one of the shortest books of the Bible – only one chapter. Study it then congratulate yourself on reading a whole book of the Bible today! He mainly reiterates themes he expressed in the Gospel of John and in 1 John.
To the chosen lady and her children, whom I love in the truth—and not I only, but also all who know the truth— 2 because of the truth, which lives in us and will be with us forever:
3 Grace, mercy and peace from God the Father and from Jesus Christ, the Father’s Son, will be with us in truth and love.
The “chosen lady and her children” probably means the church and its members, or perhaps an individual church.
4 It has given me great joy to find some of your children walking in the truth, just as the Father commanded us. 5 And now, dear lady, I am not writing you a new command but one we have had from the beginning. I ask that we love one another. 6 And this is love: that we walk in obedience to his commands. As you have heard from the beginning, his command is that you walk in love.
He ties love to obedience to God, just as he did in 1 John and the Gospel of John (John 14:21 Whoever has my commands and obeys them, he is the one who loves me. He who loves me will be loved by my Father, and I too will love him and show myself to him.”) Doing what God commands is the right way to show love for others.
7 Many deceivers, who do not acknowledge Jesus Christ as coming in the flesh, have gone out into the world. Any such person is the deceiver and the antichrist. 8 Watch out that you do not lose what you have worked for, but that you may be rewarded fully. 9 Anyone who runs ahead and does not continue in the teaching of Christ does not have God; whoever continues in the teaching has both the Father and the Son. 10 If anyone comes to you and does not bring this teaching, do not take him into your house or welcome him. 11 Anyone who welcomes him shares in his wicked work.
Also, he reiterates that Jesus is the only way (“Anyone who runs ahead and does not continue in the teaching of Christ does not have God; whoever continues in the teaching has both the Father and the Son.”). He also teaches not to associate with false teachers.
12 I have much to write to you, but I do not want to use paper and ink. Instead, I hope to visit you and talk with you face to face, so that our joy may be complete.
13 The children of your chosen sister send their greetings.
The Holy Bible : New International Version. 1996, c1984. Grand Rapids: Zondervan.