The second festival, then, after the festival of the unceasing sacrifice, is set the sacrifice of the sabbath, and every holy and just man ought to keep also the festival of the sabbath. But what is the festival of the sabbath except that of which the Apostle says: "There remaineth therefore a sabbath-keeping," that is, an observance of the sabbath, "for the people of God"? Leaving aside, therefore, the Jewish observances of the sabbath, let us see of what sort the Christian's observance of the sabbath ought to be. On the sabbath day one ought to do no work out of all the works of the world. If, therefore, you cease from all secular works and do nothing worldly, but have leisure for spiritual works, come together to the church, lend an ear to the divine readings and expositions, think upon heavenly things, take thought for the hope to come, keep the judgment that is to come before your eyes, look not to present and visible things but to invisible and future things — this is the Christian's observance of the sabbath. But these things the Jews too ought to observe. Finally, even among them, if a smith, if a builder, and if any workman of this kind there be, he is idle on the sabbath day; but the reader of the divine law or the teacher does not cease from his work, and yet does not defile the sabbath; for so the Lord too says to them: "Have you not read that the priests too in the temple profane the sabbath and are without blame?" He therefore who has ceased from the works of the world and has leisure for spiritual acts, this is he who keeps the sacrifice of the sabbath and the feast day of the sabbaths. Nor does he carry burdens on the way; for every sin is a burden, as the prophet also says: "Like a heavy burden they have weighed upon me." Nor does he kindle a fire — that fire, namely, of which it is said: "Walk in the light of your fire and in the flame which you have kindled." On the sabbath each one sits in his own place and does not go forth from it. What, then, is the place of the spiritual soul? Justice is its place, truth, wisdom, sanctification, and all that Christ is, is the place of the soul. From which place it ought not to go forth, that it may keep the true sabbaths and fulfill the feast day in the sacrifices of the Sabbaths, as the Lord also said: "He who abides in me, and I in him." But as to what we called the true sabbaths, if we seek out more deeply what the true sabbaths are, the observance of the true sabbath is beyond this world. For that which is written in Genesis, that "the Lord rested on the sabbath day from his works," we do not see either to have been done then on the seventh day or to be done even now. For we always see God working, and there is no sabbath on which God does not work, on which He does not bring forth His sun over the good and the evil and rain upon the just and the unjust, on which He does not bring forth on the mountains hay and grass for the service of men, on which He does not smite and heal, lead down to hell and bring back, on which He does not kill and make alive. Hence the Lord too in the Gospels, when the Jews laid down a rule for Him about working and healing on the sabbath, answered them: "My Father worketh until now, and I work," showing by these words that on no sabbath of this world does God rest from the dispensations of the world and from the provisions for the human race. For He indeed made the creatures from the beginning and brought forth substances, as many as He knew, being the Founder of things, could suffice for the perfection of the world, but until the consummation of the age He does not cease from their provision and dispensation. There will be, therefore, a true sabbath, in which God will rest from all His works — the age to come, then, when grief and sadness and groaning shall flee away, and God shall be all and in all. On which sabbath may God grant us too to keep a feast day with Him and to celebrate festivals with His holy angels, offering the sacrifice of praise and rendering to the Most High our vows, which here our lips have set apart. Then perhaps also the unceasing sacrifice, of which we expounded above, will be better offered. For then the soul will better be able unceasingly to stand before God and to offer the sacrifice of praise through the great high priest, who is a priest forever according to the order of Melchizedek.
Source: HOMILIES ON NUMBERS 23.4