‘Or because the signs of His Godhead were undoubted, shall the proof of his having a human body be assumed false, and thus the indications of both natures be accepted to prove Him Creator, but not be accepted for the salvation of the creature? No, for the flesh did not lessen what belongs to His Godhead, nor the Godhead destroy what belongs to His flesh. For He is at once both eternal from His Father and temporal from His mother, inviolable in His strength, passible in our weakness: in the Triune Godhead, of one and the same substance with the Father and the Holy Spirit, but in taking Manhood on Himself, not of one substance but of one and the same person (so that He was at once rich in poverty, almighty in submission, impassible in punishment, immortal in death). For the Word was not in any part of It turned either into flesh or into soul, seeing that the absolute and unchangeable nature of the Godhead is ever entire in its Essence, receiving no loss nor increase, and so beatifying the nature that It had assumed that that nature remained for ever glorified in the person of the Glorifier. (But why should it seem unsuitable or impossible that the Word and flesh and soul should be one Jesus Christ, and that the Son of God and the Son of Man should be one, if flesh and soul which are of different natures make one person even without the Incarnation of the Word: since it is much easier for the power of the Godhead to produce this union of Himself and man than for the weakness of manhood by itself to effect it in its own substance.) Therefore neither was the Word changed into flesh nor flesh into the Word: but both remains in one and one is in both, not divided by the diversity and not confounded by intermixture: He is not one by His Father and another by His mother, but the same, in one way by His Father before every beginning, and in another by His mother at the end of the ages: so that He was mediator between God and men, the man Christ Jesus (1 Timothy 2:5), in whom dwelt the fulness of the Godhead bodily (Colossians 2:9): because it was the assumed (nature) not the Assuming (nature) which was raised, because God exalted Him and gave Him the Name which is above every name: that in the name of Jesus every knee should bow, of things in heaven and things on earth and things under the earth, and that every tongue should confess that Jesus Christ the Lord is in the glory of God the Father (Philippians 2:9)’.
from Letter 35, to Julian, Bishop of Cos, by Pope St Leo I, c.400-461
O Lord Jesu Christ, who didst strengthen thy holy Bishop and Doctor Pope Leo to maintain both by word and deed the verity of thy sacred Humanity: grant, we beseech thee; that guided by the light of his doctrine, we may earnestly defend the faith of thy holy Incarnation; who livest and reignest with the Father, in the unity of the Holy Ghost, ever one God, world without end. Amen. - Divine Worship: The Missal.
Fr Lee Kenyon