The Second Epistle of John (2 John) is very short, just 13 verses. The author writes using the name of a well respected Christian in order to convince others that a particular belief is wrong. The entire epistle is about this one point. He writes against the belief that Jesus' body was spiritual in nature, and not fleshy; a belief known as docetism.2 John says that if you don't have the right beliefs about the relationship between Jesus-as-god and Jesus-as-man then you are godless (2 John 1:7-9), and Christians can't greet you politely nor welcome you in to church or home (2 John 1:10-11). Just to greet people with wrong beliefs, says 2 John, is to be in league with evil! This has no doubt helped encourage the intolerant and fundamentalist streams in Christian history. 2 John does contradict a few other verses in the Bible that say Christians should debate doctrine patiently and respectfully (1 Peter 3:15-16, 2 Timothy 4:2, Titus 3:2 and Colossians 4:6).
Some of the earliest groups of Christians were docetist but after the rise of Pauline Christianity they were forcibly silenced and mostly eradicated. Pauline Christians, who finalized the choice of what books went into the Bible, obviously then accepted 2 John as authentic, even though historians doubt its history.