26 The Apostles' belief prepares us for the understanding of this mystery; when it testifies that Jesus Christ was found in fashion as a man and was sent in the likeness of the flesh of sin. For being fashioned as a man, He is in the form of a servant, but not in the imperfections of a servant's nature; and being in the likeness of the flesh of sin, the Word is indeed flesh, but is in the likeness of the flesh of sin and not the flesh of sin itself. In like manner Jesus Christ being man is indeed human, but even thus cannot be anything else but Christ, born as man by the birth of His body, but not human in defects, as He was not human in origin. The Word made flesh could not but be the flesh that He was made; yet He remained always the Word, though He was made flesh. As the Word made flesh could not vacate the nature of His Source, so by virtue of the origin of His nature He could not but remain the Word: but at the same time we must believe that the Word is that flesh which He was made; always, however, with the reserve, that when He dwelt among us, the flesh was not the Word, but was the flesh of the Word dwelling in the flesh.
Though we have proved this, still we will see whether in the whole range of suffering, which He endured, we can anywhere detect in our Lord the weakness of bodily pain. We will put off for a time the discussion of the passages on the strength of which heresy has attributed fear to our Lord; now let us turn to the facts themselves: for His words cannot signify fear if His actions display confidence.
Source: On the Trinity (New Advent)