(from Oration on Holy Baptism, 381 A.D.)
Illumination is the splendor of souls, the conversion of the life, the question put to the Godward conscience. It is the aid to our weakness, the renunciation of the flesh, the following of the Spirit, the fellowship of the Word, the improvement of the creature, the overwhelming of sin, the participation of light, the dissolution of darkness. It is the carriage to God, the dying with Christ, the perfection of the mind, the bulwark of Faith, the key of the Kingdom of Heaven, the change of life, the removal of slavery, the loosing of chains, the remodeling of the whole man.
Why should I go into further detail? Illumination is the greatest and most magnificent of the Gifts of God. For just as we speak of the Holy of Holies, and the Song of Songs, as more comprehensible and more excellent than others, so is this called Illumination, as being more holy than any other illumination which we possess.
We call it, the Gift, the Grace, the Baptism, the Unction, Illumination, the Clothing of Immortality, the Laver of Regeneration, the Seal, and everything that is honorable. We call it the Gift, because it is given to us in return for nothing on our part; Grace, because it is conferred even on debtors; Baptism, because sin is buried with it in the water; Unction, as Priestly and Royal, for such were they who were anointed; Illumination, because of its splendor; Clothing, because it hides the shame; the Laver, because it washes us; the Seal because it preserves us, and is moreover the indication of Dominion. In it the heavens rejoice; it is glorified by angels, because of its kindred splendor. It is the image of the heavenly bliss.
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