1 John 2:19

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.

Read 1 John 2:19 Using Other Translations

They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us.
They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us.
These people left our churches, but they never really belonged with us; otherwise they would have stayed with us. When they left, it proved that they did not belong with us.