The book of 2 John is the second of three letters that are included in the New Testament of the Bible that were written by John, son of Zebedee. John also authored the gospel that bears his name, and the book of Revelation. John was very close to Jesus during His ministry on earth and is referred to as, "the disciple whom Jesus loved".
This letter was written around the same time as 1 John somewhere between a.d. 70 - 100. In this letter John warns those who might take in and provision traveling evangelists and missionaries, that not all of them are spreading the truth. The message is to be wary of those teaching heresies.