Augustine of Hippo, Against Two Letters of the Pelagians I.2: Why Heretical Writings Must Be Answered

For the new heretics, enemies of the grace of God which is given by Jesus Christ our Lord to small and great, although they are already shown more openly to need to be avoided by a manifest disapprobation, still do not cease by their writings to try the hearts of the less cautious and less learned. And these must certainly be answered, lest they should confirm themselves or their friends in that wicked error; even if we were not afraid that they might deceive some one of the catholics by their plausible discourse. But since they do not cease to growl at the entrances to the Lord’s fold, and from every side to tear open approaches with a view to tear in pieces the sheep redeemed at such a price; and since the pastoral watch-tower is common to all of us who discharge the office of the episcopate (although you are prominent therein on a loftier height), I do what I can in respect of my small portion of the charge, as the Lord condescends by the aid of your prayers to grant me power, to oppose to their pestilent and crafty writings, healing and defensive writings, so that the madness with which they are raging may either itself be cured, or may be prevented from hurting others.