Objection 1. It would seem that Christ's temptation should not have taken place after His fast. For it has been said above (Question 40, Article 2) that an austere mode of life was not becoming to Christ. But it savors of extreme austerity that He should have eaten nothing for forty days and forty nights, for Gregory (Hom. xvi inn Evang.) explains the fact that "He fasted forty days and forty nights," saying that "during that time He partook of no food whatever." It seems, therefore, that He should not thus have fasted before His temptation.
Objection 2. Further, it is written (Mark 1:13) that "He was in the desert forty days and forty nights; and was tempted by Satan." Now, He fasted forty days and forty nights. Therefore it seems that He was tempted by the devil, not after, but during, His fast.
Objection 3. Further, we read that Christ fasted but once. But He was tempted by the devil, not only once, for it is written (Luke 4:13) "that all the temptation being ended, the devil departed from Him for a time." As, therefore, He did not fast before the second temptation, so neither should He have fasted before the first.
On the contrary, It is written (Matthew 4:2-3): "When He had fasted forty days and forty nights, afterwards He was hungry": and then "the tempter came to Him."
I answer that, It was becoming that Christ should wish to fast before His temptation. First, in order to give us an example. For since we are all in urgent need of strengthening ourselves against temptation, as stated above (Article 1), by fasting before being tempted, He teaches us the need of fasting in order to equip ourselves against temptation. Hence the Apostle (2 Corinthians 6:5-7) reckons "fastings" together with the "armor of justice."
Secondly, in order to show that the devil assails with temptations even those who fast, as likewise those who are given to other good works. And so Christ's temptation took place after His fast, as also after His baptism. Hence since rather Chrysostom says (Hom. xiii super Matth.): "To instruct thee how great a good is fasting, and how it is a most powerful shield against the devil; and that after baptism thou shouldst give thyself up, not to luxury, but to fasting; for this cause Christ fasted, not as needing it Himself, but as teaching us."
Thirdly, because after the fast, hunger followed, which made the devil dare to approach Him, as already stated (1, ad 1). Now, when "our Lord was hungry," says Hilary (Super Matth. iii), "it was not because He was overcome by want of food, but because He abandoned His manhood to its nature. For the devil was to be conquered, not by God, but by the flesh." Wherefore Chrysostom too says: "He proceeded no farther than Moses and Elias, lest His assumption of our flesh might seem incredible."
Reply to Objection 1. It was becoming for Christ not to adopt an extreme form of austere life in order to show Himself outwardly in conformity with those to whom He preached. Now, no one should take up the office of preacher unless he be already cleansed and perfect in virtue, according to what is said of Christ, that "Jesus began to do and to teach" (Acts 1:1). Consequently, immediately after His baptism Christ adopted an austere form of life, in order to teach us the need of taming the flesh before passing on to the office of preaching, according to the Apostle (1 Corinthians 9:27): "I chastise my body, and bring it into subjection, lest perhaps when I have preached to others, I myself should become a castaway."
Reply to Objection 2. These words of Mark may be understood as meaning that "He was in the desert forty days and forty nights," and that He fasted during that time: and the words, "and He was tempted by Satan," may be taken as referring, not to the time during which He fasted, but to the time that followed: since Matthew says that "after He had fasted forty days and forty nights, afterwards He was hungry," thus affording the devil a pretext for approaching Him. And so the words that follow, and the angels ministered to Him, are to be taken in sequence, which is clear from the words of Matthew (4:11): "Then the devil left Him," i.e. after the temptation, "and behold angels came and ministered to Him." And as to the words inserted by Mark, "and He was with the beasts," according to Chrysostom (Hom. xiii in Matth.), they are set down in order to describe the desert as being impassable to man and full of beasts.
On the other hand, according to Bede's exposition of Mark 1:12-13, our Lord was tempted forty days and forty nights. But this is not to be understood of the visible temptations which are related by Matthew and Luke, and occurred after the fast, but of certain other assaults which perhaps Christ suffered from the devil during that time of His fast.
Reply to Objection 3. As Ambrose says on Luke 4:13, the devil departed from Christ "for a time, because, later on, he returned, not to tempt Him, but to assail Him openly"--namely, at the time of His Passion. Nevertheless, He seemed in this later assault to tempt Christ to dejection and hatred of His neighbor; just as in the desert he had tempted Him to gluttonous pleasure and idolatrous contempt of God.
Source: Summa Theologica (New Advent)