LINK: 2 John
The apostle John wrote this letter around the year 90. He addressed it to "the chosen lady," which may be an individual or may be a church. If it's a church then the greeting from "your chosen sister" would refer to the church where John was at the time of writing (probably Ephesus).
John's primary purpose is to warn his readers of false teachings and to exhort them to remain firm in the truth. The teachings of Christ are truth, and anyone who does not accept them does not belong to Him. They need to guard against anyone who brings any teaching that does not align with Christ.
One specific false teaching at that time was that of gnosticism. Gnostics taught that Jesus was not God in the flesh. (I posted more on the teachings of gnosticism here.) John warned against this particular teaching and those who taught it (verse 7).
The best way to avoid the trap of false teaching is to really know the truth and to live the truth. It's not an easy task; it requires reading, thought, discussion, and study. It's much easier to be complacent, but it's also very dangerous to be complacent. Our world is full of false teaching, and much of it is packaged in a way that sounds biblical.
The key is to examine the core of any "new" teaching. When it's boiled down, what does it say about Jesus? It doesn't have a leg to stand on unless it teaches that Jesus came as God in the flesh, born of a virgin, lived a sinless life, died on the cross as payment for man's sin, was buried, raised from the dead, ascended into heaven, and will one day return to claim those who belong to Him for eternity. In the meantime, He left us the command to love God with all our being and to love others. Those who belong to Him will keep His commandments.
Father, may we be more aware of the falsehoods of the many various teachings of our day. Teach us through Your word and Your Holy Spirit to know the truth and to recognize what does not align with the truth. May we grow in our understanding of Your word and seek to walk in truth, keeping Your commands. In the name of Jesus, amen.