1 John 2 19
They left us, but they didn't belong with us, for if they did, they would have remained with us. When they left they showed that none of them belonged with us.
Compare with other 3 random translations
ASV 1 John 2:19They 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 they might be made manifest that they all are not of us.
GW 1 John 2:19They left us. However, they were never really part of us. If they had been, they would have stayed with us. But by leaving they made it clear that none of them were part of us.
WEB 1 John 2:19They went out from us, but they didn’t belong to us; for if they had belonged to us, they would have continued with us. But they left, that they might be revealed that none of them belong to us.