They 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.
But that's just an addition to the text; you notice it's in italics. Which indicates it to be an addition to the text, and they're just trying to complete the thought, but the thought is complete enough without their addition. "Whosoever denies the Son has not the Father."