Thomas Aquinas, Compendium of Theology

Since prayer manifests desire, the nature of the prayer Christ offered when His passion was upon Him may be gathered from the different desires He expressed. In Matthew 26:39 He begs: "My Father, if it be possible, let this chalice pass from Me. Nevertheless, not as I will, but as Thou wilt." In saying, "Let this chalice pass from Me," He indicates the movement of His lower appetite and natural desire, whereby all naturally shrink from death and desire life. And in saying, "Nevertheless not as I will, but as Thou wilt," He gives expression to the movement of His higher reason, which looks on all things as comprised under the ordinations of divine wisdom. The same is the bearing of the added words, "If this chalice may not pass away" (Matt. 26:42), whereby He showed that only those events can occur which take place according to the order of the divine will.

Although the chalice of the passion did not pass from Him, but He had to drink it, we may not say that His prayer went unheard. For, as the Apostle assures us in Hebrews 5:7, in all things Christ "was heard for His reverence." Since prayer, as we have remarked, is expressive of desire, we pray unconditionally for what we wish unconditionally; and so the very desires of the just have the force of prayer with God, according to Psalm 9:17: "The Lord hath heard the desire of the poor." But we wish unconditionally only what we desire with our higher reason, which alone has the power of assenting to an undertaking. Christ prayed absolutely that the Father's will might be done, for this was what He wished absolutely. But He did not thus pray that the chalice might pass from Him, because He wished this, not absolutely, but according to His lower reason, as we have stated.