That God is so angry against idolatry, that He has even enjoined those to be slain who persuade others to sacrifice and serve idols
In Deuteronomy: “But if your brother, or your son, or your daughter, or your wife which is in your bosom, or your friend which is the fellow of your own soul, should ask you secretly, saying, Let us go and serve other gods, the gods of the nations, you shall not consent unto him, and you shall not hearken unto him, neither shall your eye spare him, neither shall you conceal him, declaring you shall declare concerning him. Your hand shall be upon him first of all to put him to death, and afterwards the hand of all the people; and they shall stone him, and he shall die, because he has sought to turn you away from the Lord your God.” And again the Lord speaks, and says, that neither must a city be spared, even though the whole city should consent to idolatry: “Or if you shall hear in one of the cities which the Lord your God shall give you, to dwell there, saying, Let us go and serve other gods, which you have not known, slaying you shall kill all who are in the city with the slaughter of the sword, and bum the city with fire, and it shall be without habitation for ever.
Moreover, it shall no more be rebuilt, that the Lord may be turned from the indignation of His anger. And He will show you mercy, and He will pity you, and will multiply you, if you will hear the voice of the Lord your God, and will observe His precepts.” Remembering which precept and its force, Mattathias slew him who had approached the altar to sacrifice. But if before the coming of Christ these precepts concerning the worship of God and the despising of idols were observed, how much more should they be regarded since Christ's advent; since He, when He came, not only exhorted us with words, but with deeds also, but after all wrongs and contumelies, suffered also, and was crucified, that He might teach us to suffer and to die by His example, that there might be no excuse for a man not to suffer for Him, since He suffered for us; and that since He suffered for the sins of others, much rather ought each to suffer for his own sins.
And therefore in the Gospel He threatens, and says: “Whosoever shall confess me before men, him will I also confess before my Father which is in heaven; but whosoever shall deny me before men, him will I also deny before my Father which is in heaven.” The Apostle Paul also says: “For if we die with Him, we shall also live with Him; if we suffer, we shall also reign with Him; if we deny Him, He also will deny us.” John too: “Whosoever denies the Son, the same has not the Father; he that acknowledges the Son, has both the Son and the Father.” Whence the Lord exhorts and strengthens us to contempt of death, saying: “Fear not them which kill the body, but are not able to kill the soul; but rather fear Him which is able to kill soul and body in Gehenna.” And again: “He that loves his life shall lose it; and he who hates his life in this world, shall keep it unto life eternal.”
Source: The Treatises of Cyprian (New Advent)