John Cassian, On the Incarnation of the Lord, Against Nestorius I.1: The heresy compared to the hydra of the poets

The tales of poets tell us that of old the hydra when its heads were cut off gained by its injuries, and sprang up more abundantly: so that owing to a miracle of a strange and unheard-of kind, its loss proved a kind of gain to the monster which was thus increased by death, while that extraordinary fecundity doubled everything which the knife of the executioner cut off, until the man who was eagerly seeking its destruction, toiling and sweating, and finding his efforts so often baffled by useless labours, added to the courage of battle the arts of craft, and by the application of fire, as they tell us, cut off with a fiery sword the manifold offspring of that monstrous body; and so when the inward parts were thus burnt, by cauterizing the rebellious throbbings of that ghastly fecundity, at length those prodigious births were brought to an end. Thus also heresies in the churches bear some likeness to that hydra which the poets’ imagination invented; for they too hiss against us with deadly tongues; and they too cast forth their deadly poison, and spring up again when their heads are cut off. But because the medicine should not be wanting when the disease revives, and because the remedy should be the more speedy as the sickness is the more dangerous, our Lord God is able to bring to pass that that may be a truth in the church’s warfare, which Gentile fictions imagined of the death of the hydra, and that the fiery sword of the Holy Spirit may cauterize the inward parts of that most dangerous birth, in the new heresy to be put down, so that at last its monstrous fecundity may cease to answer to its dying throbs.