The children of Israel served the king of Babylon for seventy years, and afterwards, were set free and returned to Jerusalem. Likewise, we ourselves must serve all of this life, either for our faults and their punishment, or at least in hardship.
For this reason, the Church, being set, as it were, in the captivity of Babylon, that is, in this world, and wishing penance to be done, so that She may someday be set free and come to the heavenly Jerusalem, keeps Septagesima (i.e. the “70th”). Therefore, She begins to read the five books of Moses, since the usefulness of penance is set out in them step by step as follows.
The first book, namely Genesis, instructs us in the first stages of penance, namely, in faith and fear, which are the essence of penance, since penance is conceived through them. It instructs us in the Faith in the same way as the Creed does, for what is said there, “of things visible and invisible”, is also said here: “In the beginning God created heaven and earth,” which is to say, the empyreal heaven, and the things which are in it, which are invisible, and the earth, that is, all these visible things. Just as in the Creed the persons of the Father and the Son are mentioned, so also in Genesis “In the beginning” (that is, in the Son,) God (that is, the Father,) created heaven and earth. Afterwards, the person of the Holy Spirit is named, when it says “And the spirit of the Lord was borne over the waters”, that is, the Holy Spirit, who created and rules over all things.
It also instructs us in the faith of the Incarnation and Passion, so that we might believe that Christ suffered in so far as he is a man, and not in so far as he is God; this is expressed through Isaac, who was not sacrificed, but rather a ram (took his place). Again it instructs us to believe that Christ was given by grace, and not for the sake of our merits, as Isaac (was given to Abraham by grace.) It also instructs us in the faith of the Resurrection and the Ascension and the sending of the Holy Spirit through the figure of Joseph, who after being sold was exalted in Egypt, distributing wheat through all of that land, just as Christ, after being sold, was exalted unto the world, and distributes the wheat of the word of God throughout the world through his preachers. …
Also, in the figure of Adam, it instills fear, lest through the vice of gluttony or through inobedience we be cast out of the spiritual Paradise, as he was cast out of the earthly Paradise. In the figure of Cain, it instructs us to guard against murder; in the cities which were completely destroyed, to stay away from the vice of Sodom; and in the flood, to abstain from every vice; and again, in the figure of Esau, to abstain from the vice of gluttony, since he was rejected he ate the red beans (i.e., the food which Jacob sold him for his birthrights). Furthermore, because in Septagesima we remember the misery which we incur because of the sins of our parents, we read the book of Genesis, which treats of the expulsion of the first parents from Paradise, … To signify how great our wretchedness is, first we read and sing (in the responsories) about the dignity of man, namely, that he was made in the image and likeness of God, that he was set in Paradise, that a companion was made for him, and that he could not die, nor suffer any other penalty, except that it came from his own fault.
Now the introit of the Mass is “The groans of death have surrounded me” in which the Church confesses that it is in suffering and afflictions because of Sin … But, lest this mourning beget sloth or sadness within us, which lead to (spiritual) death, in the verse it speaks of consolation: “I will love Thee, o Lord, my strength.” … And notice that these words (of the Introit) are also the voice of the Church of the early days, weeping enable the first martyr, whose blood cries out to the Lord from the earth, which opened up its mouth and received it from the hand of Cain, his brother. For this reason, the station is at (the tomb of) St Lawrence, whose precious death by a new and unheard-of kind of suffering cried out to heaven, and was heard in all the world; wherefore also the authority of the Roman church was declared above all others in the martyrs. (William Durandus, Rationale Divinorum Officiorum, 6, 25, 1-4)