John Calvin Commentary Deuteronomy 13:12

"If thou shalt hear tell concerning one of thy cities, which Jehovah thy God giveth thee to dwell there, saying," — Deuteronomy 13:12 (ASV)

If you shall hear it said. If impiety and rebellion were to become more widespread, Moses declares that whole cities, along with their inhabitants, should be destroyed rather than allow so great a crime to remain unpunished. From this, we can better infer how unholy is the tenderness of those who would want no punishment inflicted for the violation of God's religion.

If sedition arises in an army or nation, and its contagion spreads through the whole multitude, the severity of a just and moderate ruler usually extends no further than punishing the ringleaders; therefore, when God commands all without exception to be destroyed, the great atrocity of the crime is made apparent.

From this, also, we are warned that zeal for God’s glory is only cold among us unless true religion is considered more valuable than the preservation of a single city or people. But if so many are to be dragged to death together in crowds, the impudence of those who would disregard God’s injured majesty to spare even one person is beyond detestable, and their pity is cruelty itself.

And since we are created for no other end, and live for no other cause, than that God may be glorified in us, it is better for the whole world to perish than for people to enjoy the fruits of the earth only to contaminate it with their blasphemies.

If those who first professed Christ’s name had been inspired with such zeal, true religion would never have been overwhelmed and almost extinguished by so many corruptions.

But we must always remember what I have already said: this severity must not be used except when the true religion is suffering—a religion that is not only accepted by public authority and general opinion but is also proven true on solid grounds—so that it may be clear that we are God’s avengers against the wicked.