John Calvin on Matthew 10:21

Matthew 10:21. And the brother will deliver up the brother to death. He first gives warning what heavy calamities await them, and then adds a remarkable consideration, which sweetens all their bitterness. First, he announces that those circumstances which other men find to be the means of protection, or from which they obtain some relief, will prove to the disciples a fresh addition to their misery. Brothers, who ought to assist them when oppressed, to stretch out their hand to them amidst their distresses, and to watch over their safety, will be their mortal enemies.

It is a mistake however, to suppose that it happens to none but believers to be delivered up to death by their brethren: for it is possible that a father may pursue his son with holy zeal, “Par un zele sainct et plaisant a Dieu;” — “by a zeal that is holy and pleasing to God.” if he perceives him to have apostatized from the true worship of God; nay, the Lord enjoins us in such a case (Deuteronomy 13:9) to forget flesh and blood, and to bestow all our care on vindicating the glory of his name. “De maintenir la gloire de son nom, a fin que punition soit faite de l'outrage commis contre sa majeste;” — “to maintam the glory of his name, that punishment may be inflicted on the outrage comnntted against his majesty.” Whoever has fear and reverence for God will not spare his own relatives, but will rather choose that all of them should perish, if it be found necessary, than that the kingdom of Christ should be scattered, the doctrine of salvation extinguished, and the worship of God abolished. If our affections were properly regulated, there would be no other cause of just hatred among us.

On the other hand, as Christ not only restores the kingdom of God, and raises godliness to its full vigor, but even brings men back from ruin to salvation, nothing can be more unreasonable than that the ministers of so lovely a doctrine should be hated on his account. A thing so monstrous, and so contrary to nature, might greatly distress the minds of simple men: “Les gens simples, et d'esprit paisible;” — “simple people, and of peaceable dispositions.” but Christ foretells that it will actually take place.