They went out from us: This is John’s first clear statement about the false teachers. At one time, these teachers were part of the church community. They shared fellowship with John and with the other apostles. Later, they chose to leave that fellowship. They also formed a separate and exclusive group that followed false teachings about Jesus (see 4:1; 2 John 1:7; 3 John 1:9–10).
none of them belonged to us: True Christians would have remained in fellowship with the apostles and remained faithful to their teachings.