John Calvin Commentary Matthew 8:12

"but the sons of the kingdom shall be cast forth into the outer darkness: there shall be the weeping and the gnashing of teeth." — Matthew 8:12 (ASV)

But the children of the kingdom: Why does he call those persons children of the kingdom who were nothing less than children of Abraham? For those who are aliens from the faith have no right to be considered a part of God’s flock. I answer: Though they did not actually belong to the Church of God, yet, because they occupied a place in the Church, he allows them this designation.

Additionally, it should be observed:

1. First, that so long as the covenant of God remained in the family of Abraham, it had such force that the inheritance of the heavenly kingdom belonged specially to them. With respect to God himself, at least, they were holy branches from a holy root (Romans 11:16). The rejection of them, which afterwards followed, shows plainly enough that they belonged, at that time, to the family of God.
2. Second, that Christ does not now speak of individuals, but of the whole nation.

This was still harder to endure than the calling of the Gentiles. That the Gentiles should be admitted by a free adoption into the same body with the posterity of Abraham could scarcely be endured. But that the Jews themselves should be driven out to make way for their being succeeded by the Gentiles appeared to them altogether monstrous.

Yet Christ declares that both will happen: that God will admit strangers into the bosom of Abraham, and that he will exclude the children. There is an implied contrast in the phrase, the darkness that is without. It means that outside the kingdom of God, which is the kingdom of light, nothing but darkness reigns. By darkness, Scripture points out that dreadful anguish which can neither be expressed nor conceived in this life.⁵⁰⁵

⁵⁰⁵ “Laquelle la bouche de l'homme ne sauroit exprimer, ni ses sens comprendre en ce monde;” — “which the mouth of man cannot express, nor his senses comprehend, in this world.”;” — “which the mouth of man cannot express, nor his senses comprehend, in this world.”