9 For as God is His own eternal happiness, and is besides the bestower of happiness, so He is His own eternal light, and is also the bestower of light. He envies the good of none, for He is Himself the source of happiness to all good beings; He fears the evil of none, for the loss of all evil beings is in their being abandoned by Him. He can neither be benefited by those on whom He Himself bestows happiness, nor is He afraid of those whose misery is the doom awarded by His own judgment.
Very different, O Manichæus, is the object of your worship. You have departed from God in the pursuit of your own fancies, which of all kinds have increased and multiplied in your foolish roving hearts, drinking in through the sense of sight the light of the heavenly bodies. This light, though it too is made by God, is not to be compared to the light created in the minds of the pious, whom God brings out of darkness into light, as He brings them out of sinfulness into righteousness.
Still less can it be compared to that inaccessible light from which all kinds of light are derived. Nor is this light inaccessible to all; for "blessed are the pure in heart, for they shall see God." "God is light, and in Him is no darkness at all;" but the wicked shall not see light, as is said in Isaiah. To them the light-giving light is inaccessible. From the light comes not only the spiritual light in the minds of the pious, but also the material light, which is not denied to the wicked, but is made to rise on the evil and on the good.
Source: Reply to Faustus the Manichaean (New Advent)