“They went out from us, but they did not really belong to us, because if they had belonged to us, they would have remained with us. But they went out from us to demonstrate that all of them do not belong to us.”
1 John 2.19
John refers to false teachers. They denied that Jesus came in the flesh. Separation in teaching must result in separation in fellowship.
The church must be vigilant against false teaching. We must be aware that some will depart from the faith. We must not be disturbed when some leave us. This is as it must be.