Freud said in his essay Civilization and Its Discontents that every time two families become connected by a marriage, each of them thinks itself superior to the other. He called this sort of thinking "Narcissism of minor differences" and it is part of the human condition. We all want to be unique and in our efforts to be unique place a value on minor differences over the major and more fundamental similarities. We also are prone to become militant about these minor differences because our very identity is wrapped up in these minor differences.
Here is a short video from Portlandia that speaks to the narcissism of minor differences:
In the church that John is writing to in the letter known as 1 John is a church where the narcissism of minor differences has run amok. There is a group of people we now call Gnostics Christians and they have separated from the church John is writing to. Additionally, John is quick to point out that these Gnostic Christians were not really followers any way but in fact they are antichrists. (When you read it in 1 John 2, it sounds something of a jilted lover.)
The point being, there were some in the community who struggled with things like the doctrine of the incarnation and they came to a different conclusion than other members of the church did. For this, and perhaps many other irreconcilable differences, they left. When they left, they were called antichrist.
I know that now we can look back and believe that if one group saw the incarnation one way and others saw it another, we would say this is a minor difference. It was not to them. It was a major difference. So much so that people left the church and were branded as never really being with us anyway.
Contrary to the common story, there has never been ONE way that Christianity has ever been practiced. There was never a time when the Church thought, believed or practiced alike. John's church had people who thought Jesus was crucified on the cross and others in the church thought Jesus' crucifiction was an illusion. As a result of these minor differences, the church split. While some see this as Biblical precedent to split, I see it as a cautionary tale of how not to live as the Church.
When we have church leaders calling others antichrist and that they were never a part of us to begin with, then we really are reflecting rather than reforming the culture. 1 John has a lot of love language and some of the most eloquent prose/poetry in the New Testament to be sure. However, John is not perfect and perhaps falls prey to the narcissism of minor differences.