VI. There is no confusion of the two natures in Christ.
Although therefore from that beginning whereby in the Virgin's womb “the Word became flesh,” no sort of division ever arose between the Divine and the human substance, and through all the growth and changes of His body, the actions were of one Person the whole time, yet we do not by any mixing of them up confound those very acts which were done inseparably: and from the character of the acts we perceive what belonged to either form. For neither do His Divine acts affect His human, nor His human acts His Divine, since both concur in this way and to this very end that in their operation His twofold qualities be not absorbed the one by the other, nor His individuality doubled.
Therefore let those Christian phantom-mongers tell us, what nature of the Saviour's it was that was fastened to the wood of the Cross, that lay in the tomb, and that on the third day rose in the flesh when the stone was rolled away from the grave: or what kind of body Jesus presented to His disciples' eyes entering when the doors were shut upon them: seeing that to drive away the beholders' disbelief, He required them to inspect with their eyes and to handle with their hands the still open prints of the nails and the flesh wound of His pierced side.
But if in spite of the truth being so clear, their persistence in heresy will not abandon their position in the darkness, let them show whence they promise themselves the hope of eternal life, which no one can attain to, save through the mediator between God and man, the man Jesus Christ. For “there is not another name given to men under heaven, in which they must be saved.” Neither is there any ransoming of men from captivity, save in His blood, “who gave Himself a ransom for all:” who, as the blessed apostle proclaims, “when He was in the form of God, thought it not robbery that He was equal with God; but emptied Himself, receiving the form of a slave, being made in the likeness of men, and being found in fashion as a man He humbled Himself, being made obedient even unto death, the death of the cross.
For which reason God also exalted Him, and gave Him a name which is above every name: that in the name of Jesus every knee may bow of things in heaven, of things on the earth, and of things under the earth, and that every tongue may confess that the Lord Jesus Christ is in the glory of God the Father.”
Source: Letters (New Advent)