This is Saint Macrina teaching her younger brother Saint Gregory of Nyssa:
For evil must be altogether removed in every way from being, and, as we have said before, that which does not really exist must cease to exist at all. Since evil does not exist by its nature outside of free choice, when all choice is in God, evil will suffer a complete annihilation because no receptacle remains for it.
Recounted by Gregory in On the Soul and the Resurrection.
