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.
They would no doubt have continued with us - The words “no doubt” are supplied by our translators, but the affirmation is equally strong without them: “they would have remained with us.” This affirms, without any ambiguity or qualification, that if they had been true Christians they “would” have remained in the church; that is, they would not have apostatized. There could not be a more positive affirmation than that which is implied here, that those who are true Christians will continue to be such; or that the saints will not fall away from grace. John affirms it of these persons, that if they had been true Christians they would never have departed from the church. He makes the declaration so general that it may be regarded as a universal truth, that if “any” are truly “of us,” that is, if they are true Christians, they will continue in the church, or will never fall away. The statement is so made also as to teach that if any “do” fall away from the church, the fact is full proof that they never had any religion, for if they had had they would have remained steadfast in the church.