12 Their next clause is:— “Nor, as Hieracas, a light from a light, or a lamp with two flames, nor as if He was previously in being, and afterwards born, or created afresh, to be a Son.” Hieracas ignores the birth of the Only-begotten, and, in complete unconsciousness of the meaning of the Gospel revelations, talks of two flames from one lamp. This symmetrical pair of flames, fed by the supply of oil contained in one bowl, is His illustration of the substance of Father and Son.
It is as though that substance were something separate from Either Person, like the oil in the lamp, which is distinct from the two flames, though they depend upon it for their existence; or like the wick, of one material throughout and burning at both ends, which is distinct from the flames, yet provides them and connects them together. All this is a mere delusion of human folly, which has trusted to itself, and not to God, for knowledge. But the true faith asserts that God is born from God, as light from light, which pours itself forth without self-diminution, giving what it has yet having what it gave.
It asserts that by His birth He was what He is, for as He is so was He born; that His birth was the gift of the existing Life, a gift which did not lessen the store from which it was taken; and that They Two are One, for He, from Whom He is born, is as Himself, and He that was born has neither another source nor another nature, for He is Light from Light. It is in order to draw men's faith away from this, the true doctrine, that this lantern or lamp of Hieracas is cast in the teeth of those who confess Light from Light.
Because the phrase has been used in an heretical sense, and condemned both now and in earlier days, they want to persuade us that there is no true sense in which it can be employed. Let heresy immediately abandon these groundless fears, and refrain from claiming to be the protector of the Church's faith on the score of a reputation for zeal earned so dishonestly. For we allow nothing bodily, nothing lifeless, to have a place among the attributes of God; whatever is God is perfect God.
In Him is nothing but power, life, light, blessedness, Spirit. That nature contains no dull, material elements; being immutable, it has no incongruities within it. God, because He is God, is unchangeable; and the unchangeable God begot God. Their bond of union is not, like that of two flames, two wicks of one lamp, something outside Themselves. The birth of the Only-begotten Son from God is not a prolongation in space, but a begetting; not an extension, but Light from Light. For the unity of light with light is a unity of nature, not unbroken continuation.
Source: On the Trinity (New Advent)