The author of the book of Second John (as well as Third John) is identified only as “the elder,” but is attributed to Jesus‘ disciple John, who also wrote the John, First John, and Revelation.
This is a personal letter seemingly to an unnamed woman and her children, although the greeting could euphemistically reference a church.
The principles John shares are applicable to all who follow Jesus. Among other things, it warns to be careful of false teaching from “deceivers.”
There are many parallels between First John and Second John. The love, obedience, truth, and antichrist themes from First John are repeated in Second John.