13 Here there is somewhat for you to consider, Brethren, as a protection against blasphemies. For it is constantly said, “See you have produced certain resemblances; but the brightness which is thrown out from the fire, shines less brilliantly than the fire itself, and the image of the shrub has less proper subsistence, than that shrub of which it is the image. These instances have a resemblance, but they have not a thorough equality: wherefore they do not seem to be of the same substance.”
What then shall we say, if any one say, “The Father then is to the Son, such as the brightness is to the fire, and the image to the shrub”? See I have understood the Father to be eternal; and the Son to be coeternal with Him; nevertheless say we that He is as the brightness which is thrown out from and is less brilliant than the fire, or as the image which is reflected from and has less real existence than the shrub? No, but there is a thorough equality. “I do not believe it,” he will say, “because you have not discovered a resemblance.”
Well then, believe the Apostle, because he was able to see what I have said. For he says, “He thought it not robbery to be equal with God.” Equality is perfect likeness in every way. And what said he? “Not robbery.” Why? Because that is robbery which belongs to another.
Source: Sermons on Selected Lessons of the New Testament (New Advent)