But that a light is suddenly kindled in the soul, as by a fire leaping forth, is a fact known long ago to our Scriptures; as when the prophet said, “Light for yourselves the light of knowledge.” John also, who lived after him, said, “That which was in the Logos was life, and the life was the light of men;” which “true light lightens every man that comes into the world” (i.e., the true world, which is perceived by the understanding), and makes him a light of the world: For this light shone in our hearts, to give the light of the glorious Gospel of God in the face of Christ Jesus. And therefore that very ancient prophet, who prophesied many generations before the reign of Cyrus (for he was older than he by more than fourteen generations), expressed himself in these words: “The Lord is my light and my salvation: whom shall I fear?” and, “Your law is a lamp unto my feet, and a light unto my path;” and again, “The light of Your countenance, O Lord, was manifested towards us;” and, “In Your light we shall see light.” And the Logos, exhorting us to come to this light, says, in the prophecies of Isaiah: “Enlighten yourself, enlighten yourself, O Jerusalem; for your light has come, and the glory of the Lord is risen upon you.” The same prophet also, when predicting the advent of Jesus, who was to turn away men from the worship of idols, and of images, and of demons, says, “To those that sat in the land and shadow of death, upon them has the light arisen;” and again, “The people that sat in darkness saw a great light.” Observe now the difference between the fine phrases of Plato respecting the “chief good,” and the declarations of our prophets regarding the “light” of the blessed; and notice that the truth as it is contained in Plato concerning this subject did not at all help his readers to attain to a pure worship of God, nor even himself, who could philosophize so grandly about the “chief good,” whereas the simple language of the holy Scriptures has led to their honest readers being filled with a divine spirit; and this light is nourished within them by the oil, which in a certain parable is said to have preserved the light of the torches of the five wise virgins.
Source: Against Celsus (New Advent)