24 Consider now with attention these three most sacred days, the days signalized by the Lord's crucifixion, rest in the grave, and resurrection. Of these three, that of which the cross is the symbol is the business of our present life: those things which are symbolized by His rest in the grave and His resurrection we hold by faith and hope. For now the command is given to each man, “Take up your cross, and follow me.” But the flesh is crucified, when our members which are upon the earth are mortified, such as fornication, uncleanness, luxury, avarice, etc., of which the apostle says in another passage: “If you live after the flesh, you shall die; but if you through the Spirit do mortify the deeds of the body, you shall live.” Hence also he says of himself: “The world is crucified unto me, and I unto the world.” And again: “Knowing this, that our old man is crucified with Him, that the body of sin might be destroyed, that henceforth we should not serve sin.” The period during which our labours tend to the weakening and destruction of the body of sin, during which the outward man is perishing, that the inward man may be renewed day by day—that is the period of the cross.
25. These are, it is true, good works, having rest for their recompense, but they are meanwhile laborious and painful: therefore we are told to be “rejoicing in hope,” that while we contemplate the future rest, we may labour with cheerfulness in present toil. Of this cheerfulness the breadth of the cross in the transverse beam to which the hands were nailed is an emblem: for the hands we understand to be symbolic of working, and the breadth to be symbolic of cheerfulness in him who works, for sadness straitens the spirit. In the height of the cross, against which the head is placed, we have an emblem of the expectation of recompense from the sublime justice of God, “who will render to every man according to his deeds; to them who, by patient continuance in well-doing, seek for glory, and honour, and immortality, eternal life.” Therefore the length of the cross, along which the whole body is extended, is an emblem of that patient continuance in the will of God, on account of which those who are patient are said to be long-suffering. The depth also, i.e. the part which is fixed in the ground, represents the occult nature of the holy mystery. For you remember, I suppose, the words of the apostle, which in this description of the cross I aim at expounding: “That ye, being rooted and grounded in love, may be able to comprehend with all saints what is the breadth, and length, and depth, and height.”
Those things which we do not yet see or possess, but hold in faith and hope, are the things represented in the events by which the second and third of the three memorable days above mentioned were signalized [viz. the Lord's rest in the grave, and His resurrection]. But the things which keep us occupied in this present life, while we are held fast in the fear of God by the commandments, as by nails driven through the flesh (as it is written, “Make my flesh fast with nails by fear of You”), are to be reckoned among things necessary, not among those which are for their own sakes to be desired and coveted. Hence Paul says that he desired, as something far better, to depart and to be with Christ: “nevertheless,” he adds, “to remain in the flesh is expedient for you” — necessary for your welfare. This departing and being with Christ is the beginning of the rest which is not interrupted, but glorified by the resurrection; and this rest is now enjoyed by faith, “for the just shall live by faith.” “Do you not know,” says the same apostle, “that so many of us as were baptized into Jesus Christ, were baptized into His death? Therefore we are buried with Him by baptism unto death.” How? By faith. For this is not actually completed in us so long as we are still “groaning within ourselves, and waiting for the adoption, to wit, the redemption of our body: for we are saved by hope; but hope that is seen is not hope: for what a man sees why does he yet hope for? But if we hope for that we see not, then do we with patience wait for it.”
26. Remember how often I repeat this to you, that we are not to think that we ought to be made happy and free from all difficulties in this present life, and are therefore at liberty to murmur profanely against God when we are straitened in the things of this world, as if He were not performing what He promised. He has indeed promised the things which are necessary for this life, but the consolations which mitigate the misery of our present lot are very different from the joys of those who are perfect in blessedness. “In the multitude of my thoughts within me,” says the believer, “Your comforts, O Lord, delight my soul.” Let us not therefore murmur because of difficulties; let us not lose that breadth of cheerfulness, of which it is written, “Rejoicing in hope,” because this follows—“patient in tribulation.” The new life, therefore, is meanwhile begun in faith, and maintained by hope: for it shall only then be perfect when this mortal shall be swallowed up in life, and death swallowed up in victory; when the last enemy, death, shall be destroyed; when we shall be changed, and made like the angels: for “we shall all rise again, but we shall not all be changed.” Again, the Lord says, “They shall be equal unto the angels.” We now are apprehended by Him in fear by faith: then we shall apprehend Him in love by sight. For “while we are at home in the body, we are absent from the Lord: for we walk by faith, not by sight.” Hence the apostle himself, who says, “I follow after, if that I may apprehend that for which also I am apprehended of Christ Jesus,” confesses frankly that he has not attained to it. “Brethren,” he says, “I count not myself to have apprehended.” Since, however, our hope is sure, because of the truth of the promise, when he said elsewhere, “Therefore we are buried with Him by baptism into death,” he adds these words, “that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life.” We walk, therefore, in actual labour, but in hope of rest, in the flesh of the old life, but in faith of the new. For he says again: “The body is dead because of sin; but the spirit is life because of righteousness. But if the Spirit of Him that raised up Jesus from the dead dwell in you, He that raised up Christ from the dead shall also quicken your mortal bodies by His Spirit that dwells in you.”
27. Both the authority of the Divine Scriptures and the consent of the whole Church spread throughout the world have combined to ordain the annual commemoration of these things at Easter, by observances which are, as you now see, full of spiritual significance. From the Old Testament Scriptures we are not taught as to the precise day of holding Easter, beyond the limitation to the period between the 14th and 21st days of the first month; but because we know from the Gospel beyond doubt which days of the week were signalized in succession by the Lord's crucifixion, His resting in the grave, and His resurrection, the observance of these days has been enjoined in addition by Councils of the Fathers, and the whole Christian world has arrived unanimously at the persuasion that this is the proper mode of observing Easter.
Source: Letters (New Advent)