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Here’s what Aquinas had to say about the commandment to keep holy the sabbath:
This is the Third Commandment of the law, and very suitably is it so. For we are first commanded to adore God in our hearts, and the Commandment is to worship one God: “You shall not have strange gods before Me.” In the Second Commandment we are told to reverence God by word: “You shall not take the name of the Lord your God in vain.” The Third commands us to reverence God by act. It is: “Remember that you keep holy the Sabbath day”. God wished that a certain day be set aside on which men direct their minds to the service of the Lord.
There are five reasons for this Commandment. The first reason was to put aside error, for the Holy Spirit saw that in the future some men would say that the world had always existed. “In the last days there shall come deceitful scoffers, walking after their own lusts, saying: Where is His promise or His coming? For since the time that the fathers slept, all things continue as they were from the beginning of creation. For this they are willfully ignorant of, that the heavens were before, and the earth out of water, and through water, created by the word of God” [2 Pet 3:3-5]. God, therefore, wished that one day should be set aside in memory of the fact that He created all things in six days, and that on the seventh day He rested from the creation of new creatures. This is why the Lord placed this Commandment in the law, saying: “Remember that you keep holy the Sabbath day.” The Jews kept holy the Sabbath in memory of the first creation; but Christ at His coming brought about a new creation. For by the first creation an earthly man was created, and by the second a heavenly man was formed: “For in Christ Jesus neither circumcision is worth any thing, nor uncircumcision, but a new creature” [Gal 6:15]. This new creation is through grace, which came by the Resurrection: “That as Christ is risen from the dead by the glory of the Father, so we also may walk in newness of life. For if we have been planted together in the likeness of His death, so shall we also be in the likeness of His resurrection” [Rm 6:4-5]. And thus, because the Resurrection took place on Sunday, we celebrate that day, even as the Jews observed the Sabbath on account of the first creation.
The second reason for this Commandment is to instruct us in our faith in the Redeemer. For the flesh of Christ was not corrupted in the sepulchre, and thus it is said: “Moreover My flesh also shall rest in hope” [Ps 15:9]. “Nor will You let your holy one see corruption” [Ps 15:10]. Wherefore, God wished that the Sabbath should be observed, and that just as the sacrifices of the Old Law signified the death of Christ, so should the quiet of the Sabbath signify the rest of His body in the sepulchre. But we do not now observe these sacrifices, because with the advent of the reality and the truth, figures of it must cease, just as the darkness is dispelled with the rising of the sun. Nevertheless, we keep the Saturdays in veneration of the Blessed Virgin, in whom remained a firm faith on that Saturday while Christ was dead.
The third reason is that this Commandment was given to strengthen and foreshadow the fulfillment of the promise of rest. For rest indeed was promised to us: “And on that day God shall give you rest from your labor, from your vexation, and from the hard bondage, to which you had been subjugated” [Is 14:3]. “My people shall dwell in a peaceful land, in secure accommodation, and in quiet places of rest” [Is 32:18].
We hope for rest from three things: from the labors of the present life, from the struggles of temptations, and from the servitude of the devil. Christ promised this rest to all those who will come to Him: “Come to Me, all ye that labor and are burdened, and I will refresh you. Take up My yoke upon you, and learn of Me, because I am meek and humble of heart; and you shall find rest to your souls. For My yoke is sweet and My burden light” [Mt 11:28-30]
However, the Lord, as we know, worked for six days and on the seventh He rested, because it is necessary to do a perfect work: “Behold with your eyes how I have labored a little, and have found much rest to Myself” [Sir 51:35]. For the period of eternity exceeds the present time incomparably more than a thousand years exceeds one day.
Fourthly, this Commandment was given for the increase of our love: “For the corruptible body is a load upon the soul” [Wis 9:15]. And man always tends downwards towards earthly things unless he takes means to raise himself above them. It is indeed necessary to have a certain time for this; in fact, some do this continually: “I will bless the Lord at all times, His praise shall ever be in my mouth” [Ps 33:2]. And again: “Pray without ceasing” [1 Thes 5:17]. These shall enjoy the everlasting Sabbath. There are others who do this (i.e., excite love for God) during a certain portion of the day: “Seven times a day I have given praise to You” [Ps 118:164]. And some, in order to avoid being entirely apart from God, find it necessary to have a fixed day, lest they become too lukewarm in their love of God: “If you call the Sabbath delightful… then shall you delight in the Lord” [Is 58:13-14]. Again: “Then shall you abound in delights of the Almighty, and shall lift up your face to God” [Job 22:26]. And accordingly this day is not set aside for the sole exercise of games, but to praise and pray to the Lord God. Wherefore, St. Augustine says that it is a lesser evil to plough than to play on this day.
Lastly, we are given this Commandment in order to exercise works of kindliness to those who are subject to us. For some are so cruel to themselves and to others that they labor ceaselessly all on account of money. This is true especially of the Jews, who are most avaricious. “Observe the day of the Sabbath to sanctify it… that your man-servant and your maid-servant may rest, even as thyself” . This Commandment, therefore, was given for all these reasons.