Objection 1. It would seem that Christ's prayer was not always heard. For He besought that the chalice of His passion might be taken from Him, as we read (Matthew 26:39): and yet it was not taken from Him. Therefore it seems that not every prayer of His was heard.
Objection 2. Further, He prayed that the sin of those who crucified Him might be forgiven, as is related (Luke 23:34). Yet not all were pardoned this sin, since the Jews were punished on account thereof. Therefore it seems that not every prayer of His was heard.
Objection 3. Further, our Lord prayed for them "who would believe in Him through the word" of the apostles, that they "might all be one in Him," and that they might attain to being with Him (John 17:20-24). But not all attain to this. Therefore not every prayer of His was heard.
Objection 4. Further, it is said (Psalm 21:3) in the person of Christ: "I shall cry by day, and Thou wilt not hear." Not every prayer of His, therefore, was heard.
On the contrary, The Apostle says (Hebrews 5:7): "With a strong cry and tears offering up prayers... He was heard for His reverence."
I answer that, As stated above (Article 1), prayer is a certain manifestation of the human will. Wherefore, then is the request of one who prays granted, when his will is fulfilled. Now absolutely speaking the will of man is the will of reason; for we will absolutely that which we will in accordance with reason's deliberation. Whereas what we will in accordance with the movement of sensuality, or even of the simple will, which is considered as nature is willed not absolutely but conditionally [secundum quid]--that is, provided no obstacle be discovered by reason's deliberation. Wherefore such a will should rather be called a "velleity" than an absolute will; because one would will [vellet] if there were no obstacle.
But according to the will of reason, Christ willed nothing but what He knew God to will. Wherefore every absolute will of Christ, even human, was fulfilled, because it was in conformity with God; and consequently His every prayer was fulfilled. For in this respect also is it that other men's prayers are fulfilled, in that their will is in conformity with God, according to Romans 8:27: "And He that searcheth the hearts knoweth," that is, approves of, "what the Spirit desireth," that is, what the Spirit makes the saints to desire: "because He asketh for the saints according to God," that is, in conformity with the Divine will.
Reply to Objection 1. This prayer for the passing of the chalice is variously explained by the Saints. For Hilary (Super Matth. 31) says: "When He asks that this may pass from Him, He does not pray that it may pass by Him, but that others may share in that which passes on from Him to them; So that the sense is: As I am partaking of the chalice of the passion, so may others drink of it, with unfailing hope, with unflinching anguish, without fear of death."
Or according to Jerome (on Matthew 26:39): "He says pointedly, 'This chalice,' that is of the Jewish people, who cannot allege ignorance as an excuse for putting Me to death, since they have the Law and the Prophets, who foretold concerning Me."
Or, according to Dionysius of Alexandria (De Martyr. ad Origen 7): "When He says 'Remove this chalice from Me,' He does not mean, 'Let it not come to Me'; for if it come not, it cannot be removed. But, as that which passes is neither untouched nor yet permanent, so the Saviour beseeches, that a slightly pressing trial may be repulsed."
Lastly, Ambrose, Origen and Chrysostom say that He prayed thus "as man," being reluctant to die according to His natural will.
Thus, therefore, whether we understand, according to Hilary, that He thus prayed that other martyrs might be imitators of His Passion, or that He prayed that the fear of drinking His chalice might not trouble Him, or that death might not withhold Him, His prayer was entirely fulfilled. But if we understand that He prayed that He might not drink the chalice of His passion and death; or that He might not drink it at the hands of the Jews; what He besought was not indeed fulfilled, because His reason which formed the petition did not desire its fulfilment, but for our instruction, it was His will to make known to us His natural will, and the movement of His sensuality, which was His as man.
Reply to Objection 2. Our Lord did not pray for all those who crucified Him, as neither did He for all those who would believe in Him; but for those only who were predestinated to obtain eternal life through Him. Wherefore the reply to the third objection is also manifest.
Reply to Objection 4. When He says: "I shall cry and Thou wilt not hear," we must take this as referring to the desire of sensuality, which shunned death. But He is heard as to the desire of His reason, as stated above.
Source: Summa Theologica (New Advent)