Deuteronomy 13:9

“But thou shalt surely kill him, etc.] Not privately and secretly, when and where he entices, nor the enticed himself by his own authority, but after being examined, judged, and condemned by the civil magistrate; and none might judge a false prophet but the sanhedrim at Jerusalem, the sanhedrim of seventy one f127; (see <421333>Luke 13:33), but the difficulty is how such an one could be convicted, since the affair was transacted secretly, (<051306>Deuteronomy 13:6) and there were none present to be witnesses, none but the enticer and the enticed; so that either the enticer must be brought to a confession of his guilt, or the testimony of the enticed alone must be taken. The Jewish doctors say…”

“thou shalt surely kill him--not hastily, or in a private manner, but after trial and conviction; and his relative, as informer, was to cast the first stone (see on Deu 17:2; Act 7:58). It is manifest that what was done in secret could not be legally proved by a single informer; and hence Jewish writers say that spies were set in some private part of the house, to hear the conversation and watch the conduct of a person suspected of idolatrous tendencies.”

“Thou shalt kill him - Not privately, which pretence would have opened the door to innumerable murders, but by procuring his death by the sentence of the magistrate. Thou shalt cast the first stone at him, as the witness was to do. Deuteronomy 13:13”

“13:9 Strike the first blow yourself: Because the guilty parties would be members of one’s own family (13:6), this difficult demand would test the depth of a person’s commitment to worship the Lord exclusively. Love for God must take priority over love for family members and friends (Matt 10:34-39).”

“But: Deu 17:2-7; Mat 10:37; Luk 14:26 thine hand: Deu 17:7; Joh 8:7; Act 7:58 Deuteronomy 13:10”

“To such persuasion Israel was not to yield, nor were they to spare the tempters. The accumulation of synonyms (pity, spare, conceal) serves to make the passage more emphatic. כּסּה, to cover, i.e., to keep secret, conceal. They were to put him to death without pity, viz., to stone him (cf. Lev 20:2). That the execution even in this case was to be carried out by the regular authorities, is evident from the words, "thy hand shall be first against him to put him to death, and the hand of all the people afterwards," which presuppose the judicial procedure prescribed in Deu 17:7, that the witnesses were to cast the first stones at the person condemned. Deuteronomy 13:12”