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.
That they might be made manifest He shews that trial is useful and necessary for the Church. It hence follows, on the other hand, that there is no just cause for perturbation. Since the Church is like a threshing-floor, the chaff must be blown away that the pure wheat may remain. This is what God does, when he casts out hypocrites from the Church, for he then cleanses it from refuse and filth. 