15 But since he has said that the Lord has prepared not arrows only, but “instruments of death” too, in the bow, it may be asked, what are “instruments of death”? Are they, per-adventure, heretics? For they too, out of the same bow, that is, out of the same Scriptures, light upon souls not to be inflamed with love but destroyed with poison: which does not happen but after their deserts: wherefore even this dispensation is to be assigned to the Divine Providence, not that it makes men sinners, but that it orders them after they have sinned.
For through sin reaching them with an ill purpose, they are forced to understand them ill, that this should be itself the punishment of sin: by whose death, nevertheless, the sons of the Catholic Church are, as it were by certain thorns, so to say, aroused from slumber, and make progress toward the understanding of the holy Scriptures. “For there must be also heresies, that they which are approved,” he says, “may be made manifest among you:” that is, among men, seeing they are manifest to God.
Or has He haply ordained the same arrows to be at once instruments of death for the destruction of unbelievers, and wrought them burning, or for the burning, for the exercising of the faithful? For that is not false that the Apostle says, “To the one we are the savour of life unto life, to the other the savour of death unto death; and who is sufficient for these things?” It is no wonder then if the same Apostles be both instruments of death in those from whom they suffered persecution, and fiery arrows to inflame the hearts of believers.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)