You can also see hints in the letters and Gospel of John of the situation around the area of Ephesus that was unfolding at the end of the first century of Christianity. A key hint appears in 1 John 2:19: "They went out from us, but they did not really belong to us. For if they had belonged to us, they would have remained with us; but their going showed that none of them belonged to us."
Here is clear evidence of a "church split." Some group had sojourned with John's group for a time, but then had left. The entire sermon of 1 John seems to have been written to help John's church at Ephesus work through what had happened. We wish we had more information about the make-up of Christianity at Ephesus in the late first century, but it is reasonable to think that there was more than one group of believers there.
First, of course, there was the non-Christian Jewish community. Many experts on John think that the story of the blind man in John 9 was as much about what was going on in Ephesus at the time the Gospel of John was written as about the time of Jesus. In particular, the threat to kick people out of the synagogue who believed Jesus was the Messiah (John 9:22) was probably something going on in the synagogues of Ephesus. Jews were being forced to choose between their faith in Jesus and their membership in the mainstream Jewish community.
Some of the non-believing Jews at Ephesus were apparently followers of John the Baptist's teaching who did not believe Jesus was the one John said was coming. Acts gives us evidence of such a group in the late 50s there (Acts 18:24-26; 19:1-7), and the way the Gospel of John presents John the Baptist suggests the group was still alive and well as late as the 90s. 
We can imagine that there were still churches at Ephesus in the late first century that had descended from Paul's time there. They would be mostly Gentile believers who probably had copies of Paul's letters and whose thinking had a Pauline character. Perhaps there were also fiery, apocalyptic Jewish Christians like John of Revelation there.
Then there were the two groups that 1 John and the Gospel of John seem to engage. First, there is John's group. They show all the signs of a somewhat small, withdrawn community. John 6:66 feels like it is about much more than an event in the ministry of Jesus. It seems very likely that we are hearing about what happened to John's own community, when many left.
What was this group that left? The best guess is that they were an early group of "Gnostics" and a group that would be known as the "Docetists" in particular. Gnosticism was a movement that arose in the late first century AD both within Judaism and Christianity. Gnostics believed that the physical world was not only inferior to the heavenly, spiritual world but that the materials of the world were intrinsically evil. 
Accordingly, Jesus could not have been without sin if he had a body. The "Docetists" were so called because they believed that Jesus had only seemed (dokeo) to take on a human body...
 The Gospel of John consistently downplays the significance of John the Baptist in relation to Jesus in a way the other Gospels do not. So John never mentions Jesus being baptized and John uniquely tells of Jesus' disciples baptizing at the same time as John (John 3:22-30). "He must become greater; I must become less" (3:30). Similarly, John does not even accept that he is Elijah in John 1:19-21.
 A famous Gnostic Christian from about 150, Marcion, actually went so far as to say that the creator of the world, the God of the Old Testament, was an evil god and a different God than the Father of our Lord Jesus.