Objection 1. It would seem that Christ did not conform His conduct to the Law. For the Law forbade any work whatsoever to be done on the Sabbath, since God "rested on the seventh day from all His work which He had done." But He healed a man on the Sabbath, and commanded him to take up his bed. Therefore it seems that He did not conform His conduct to the Law.
Objection 2. Further, what Christ taught, that He also did, according to Acts 1:1: "Jesus began to do and to teach." But He taught (Matthew 15:11) that "not" all "that which goeth into the mouth defileth a man": and this is contrary to the precept of the Law, which declared that a man was made unclean by eating and touching certain animals, as stated Leviticus 11. Therefore it seems that He did not conform His conduct to the Law.
Objection 3. Further, he who consents to anything is of the same mind as he who does it, according to Romans 1:32: "Not only they that do them, but they also that consent to them that do them." But Christ, by excusing His disciples, consented to their breaking the Law by plucking the ears of corn on the Sabbath; as is related Matthew 12:1-8. Therefore it seems that Christ did not conform His conduct to the Law.
On the contrary, It is written (Matthew 5:17): "Do not think that I am come to destroy the Law or the Prophets." Commenting on these words, Chrysostom says: "He fulfilled the Law... in one way, by transgressing none of the precepts of the Law; secondly, by justifying us through faith, which the Law, in the letter, was unable to do."
I answer that, Christ conformed His conduct in all things to the precepts of the Law. In token of this He wished even to be circumcised; for the circumcision is a kind of protestation of a man's purpose of keeping the Law, according to Galatians 5:3: "I testify to every man circumcising himself, that he is a debtor to do the whole Law."
And Christ, indeed, wished to conform His conduct to the Law, first, to show His approval of the Old Law. Secondly, that by obeying the Law He might perfect it and bring it to an end in His own self, so as to show that it was ordained to Him. Thirdly, to deprive the Jews of an excuse for slandering Him. Fourthly, in order to deliver men from subjection to the Law, according to Galatians 4:4-5: "God sent His Son... made under the Law that He might redeem them who were under the Law."
Reply to Objection 1. Our Lord excuses Himself from any transgression of the Law in this matter, for three reasons. First, the precept of the hallowing of the Sabbath forbids not Divine work, but human work: for though God ceased on the seventh day from the creation of new creatures, yet He ever works by keeping and governing His creatures. Now that Christ wrought miracles was a Divine work: hence He says (John 5:17): "My Father worketh until now; and I work."
Secondly, He excuses Himself on the ground that this precept does not forbid works which are needful for bodily health. Wherefore He says (Luke 13:15): "Doth not every one of you on the Sabbath-day loose his ox or his ass from the manger, and lead them to water?" And farther on (Luke 14:5): "Which of you shall have an ass or an ox fall into a pit, and will not immediately draw him out on the Sabbath-day?" Now it is manifest that the miraculous works done by Christ related to health of body and soul.
Thirdly, because this precept does not forbid works pertaining to the worship of God. Wherefore He says (Matthew 12:5): "Have ye not read in the Law that on the Sabbath-days the priests in the Temple break the Sabbath, and are without blame?" And (John 7:23) it is written that a man receives circumcision on the Sabbath-day. Now when Christ commanded the paralytic to carry his bed on the Sabbath-day, this pertained to the worship of God, i.e. to the praise of God's power. And thus it is clear that He did not break the Sabbath: although the Jews threw this false accusation in His face, saying (John 9:16): "This man is not of God, who keepeth not the Sabbath."
Reply to Objection 2. By those words Christ wished to show that man is made unclean as to his soul, by the use of any sort of foods considered not in their nature, but only in some signification. And that certain foods are in the Law called "unclean" is due to some signification; whence Augustine says (Contra Faust. vi): "If a question be raised about swine and lambs, both are clean by nature, since 'all God's creatures are good'; but by a certain signification lambs are clean and swine unclean."
Reply to Objection 3. The disciples also, when, being hungry, they plucked the ears of corn on the Sabbath, are to be excused from transgressing the Law, since they were pressed by hunger: just as David did not transgress the Law when, through being compelled by hunger, he ate the loaves which it was not lawful for him to eat.
Source: Summa Theologica (New Advent)