De Synodis 54

IV. "And if any one hearing this text, For as the Father hath life in Himself so also He hath given to the Son to have life in Himself; denies that the Son is like the Father even in essence, though He testifies that it is even as He has said; let him be anathema. For it is plain that since the life which is understood to exist in the Father signifies substance, and the life of the Only-begotten which was begotten of the Father is also understood to mean substance or essence, He there signifies a likeness of essence to essence." 16. With the Son's origin as thus stated is connected the perfect birth of the undivided nature. For what in each is life, that in each is signified by essence. And in the life which is begotten of life, i.e. in the essence which is born of essence, seeing that it is not born unlike (and that because life is of life), He keeps in Himself a nature wholly similar to His original, because there is no diversity in the likeness of the essence that is born and that begets, that is, of the life which is possessed and which has been given. For though God begat Him of Himself, in likeness to His own nature, He in whom is the unbegotten likeness did not relinquish the property of His natural substance. For He only has what He gave; and as possessing life He gave life to be possessed. And thus what is born of essence, as life of life, is essentially like itself, and the essence of Him who is begotten and of Him who begets admits no diversity or unlikeness.