From St. Maximus the Confessor:
God as absolute existence, goodness, and wisdom (or rather, to speak more properly, as transcending all these things) has no contrary quality whatever. But creatures, because they all have existence, and rational and intelligent ones their aptitude for goodness and wisdom by participation and grace, do have contrary qualities. To existence is opposed nonexistence, to the aptitude for goodness and wisdom is opposed vice and ignorance. For them to exist forever or not to exist is in the power of their maker. To share in his goodness or wisdom or not to share depends on the will of rational beings.
When the Greek philosophers affirm that the substance of beings coexisted eternally with God and that they received only their individual qualities from him, they say that there is nothing contrary to substance but that opposition is found only in the qualities. We maintain, however, that the divine substance alone has no contrary because it is eternal and infinite and bestows eternity on the other substances; furthermore that nonbeing is the contrary of the substance of beings and that their eternal being or nonbeing lies in the power of the one who properly is being, ‘and his gifts are not subject to revision.’ And therefore it both always is and will be sustained by his all-powerful might even though it has nonbeing as its opposite, as was said, since it was brought into being from nonbeing by God and whether it has being or nonbeing depends on his will.
Just as evil is the privation of good and ignorance that of knowledge, so is nonbeing the privation of being–but not of being properly so called, for it has no contrary–but of true being by participation. Privations of the former depend on the will of creatures; privations of the latter depend on the will of the Creator, who out of goodness ever wills his creatures to exist and receive benefits from him.
(Four Hundred Chapters on Love 3.27-29)