Objection 1. It would seem that there was no fear in Christ. For it is written (Proverbs 28:1): "The just, bold as a lion, shall be without dread." But Christ was most just. Therefore there was no fear in Christ.
Objection 2. Further, Hilary says (De Trin. x): "I ask those who think thus, does it stand to reason that He should dread to die, Who by expelling all dread of death from the Apostles, encouraged them to the glory of martyrdom?" Therefore it is unreasonable that there should be fear in Christ.
Objection 3. Further, fear seems only to regard what a man cannot avoid. Now Christ could have avoided both the evil of punishment which He endured, and the evil of fault which befell others. Therefore there was no fear in Christ.
On the contrary, It is written (Mark 14:33): Jesus "began to fear and to be heavy."
I answer that, As sorrow is caused by the apprehension of a present evil, so also is fear caused by the apprehension of a future evil. Now the apprehension of a future evil, if the evil be quite certain, does not arouse fear. Hence the Philosopher says (Rhet. ii, 5) that we do not fear a thing unless there is some hope of avoiding it. For when there is no hope of avoiding it the evil is considered present, and thus it causes sorrow rather than fear. Hence fear may be considered in two ways. First, inasmuch as the sensitive appetite naturally shrinks from bodily hurt, by sorrow if it is present, and by fear if it is future; and thus fear was in Christ, even as sorrow. Secondly, fear may be considered in the uncertainty of the future event, as when at night we are frightened at a sound, not knowing what it is; and in this way there was no fear in Christ, as Damascene says (De Fide Orth. iii, 23).
Reply to Objection 1. The just man is said to be "without dread," in so far as dread implies a perfect passion drawing man from what reason dictates. And thus fear was not in Christ, but only as a propassion. Hence it is said (Mark 14:33) that Jesus "began to fear and to be heavy," with a propassion, as Jerome expounds (Matthew 26:37).
Reply to Objection 2. Hilary excludes fear from Christ in the same way that he excludes sorrow, i.e. as regards the necessity of fearing. And yet to show the reality of His human nature, He voluntarily assumed fear, even as sorrow.
Reply to Objection 3. Although Christ could have avoided future evils by the power of His Godhead, yet they were unavoidable, or not easily avoidable by the weakness of the flesh.
Source: Summa Theologica (New Advent)