De Synodis 1211

XI. "And if any one says that the Father is older in time than His Only-begotten Son, and that the Son is younger than the Father: let him be anathema." 25. The essential likeness conformed to the Father's essence in kind is also taught to be identical in time: lest He who is the image of God, who is the Word, who is God with God in the beginning, who is like the Father, by the insertion of times between Himself and the Father should not have in Himself in perfection that which is both image, and Word, and God. For if He be proclaimed to be younger in time, He has lost the truth of the image and likeness: for that is no longer likeness which is found to be dissimilar in time. For that very fact that God is Father prevents there being any time in which He was not Father: consequently there can be no time in the Son's existence in which He was not Son. Wherefore we must neither call the Father older than the Son nor the Son younger than the Father: for the true meaning of neither name can exist without the other.