They went out from us, but they did not belong to us. For if they had belonged to us, they would have remained with us. But their departure made it clear that none of them belonged to us.
KJV
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.
ASV
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 they might be made manifest that they all are not of us.