29 The fiftieth day is also commended to us in Scripture; and not only in the Gospel, by the fact that on that day the Holy Spirit descended, but also in the books of the Old Testament. For in them we learn, that after the Jews observed the first passover with the slaying of the lamb as appointed, 50 days intervened between that day and the day on which upon Mount Sinai there was given to Moses the Law written with the finger of God; and this “finger of God” is in the Gospels most plainly declared to signify the Holy Spirit: for where one evangelist quotes our Lord's words thus, “I with the finger of God cast out devils,” another quotes them thus, “I cast out devils by the Spirit of God.” Who would not prefer the joy which these divine mysteries impart, when the light of healing truth beams from them on the soul to all the kingdoms of this world, even though these were held in perfect prosperity and peace? May we not say, that as the two seraphim answer each other in singing the praise of the Most High, “Holy, holy, holy is the Lord God of Hosts,” so the Old Testament and the New, in perfect harmony, give forth their testimony to sacred truth? The lamb is slain, the passover is celebrated, and after 50 days the Law is given, which inspires fear, written by the finger of God. Christ is slain, being led as a lamb to the slaughter as Isaiah testifies; the true Passover is celebrated; and after 50 days is given the Holy Spirit, who is the finger of God, and whose fruit is love, and who is therefore opposed to men who seek their own, and consequently bear a grievous yoke and heavy burden, and find no rest for their souls; for love “seeks not her own.” Therefore there is no rest in the unloving spirit of heretics, whom the apostle declares guilty of conduct like that of the magicians of Pharaoh, saying, “Now as Jannes and Jambres withstood Moses, so do these also resist the truth: men of corrupt minds, reprobate concerning the faith. But they shall proceed no further: for their folly shall be manifest to all men, as theirs also was.” For because through this corruptness of mind they were utterly disquieted, they failed at the third miracle, confessing that the Spirit of God which was in Moses was opposed to them: for in owning their failure, they said, “This is the finger of God.” The Holy Spirit, who shows Himself reconciled and gracious to the meek and lowly in heart, and gives them rest, shows Himself an inexorable adversary to the proud and haughty, and vexes them with disquiet. Of this disquiet those despicable insects were a figure, under which Pharaoh's magicians owned themselves foiled, saying, “This is the finger of God.”
30. Read the book of Exodus, and observe the number of days between the first passover and the giving of the Law. God speaks to Moses in the desert of Sinai on the first day of the third month. Mark, then, this as one day of the month, and then observe what (among other things) the Lord said on that day: “Go unto the people, and sanctify them today and tomorrow, and let them wash their clothes, and be ready against the third day; for the third day the Lord will come down in the sight of all the people upon Mount Sinai.” The Law was accordingly given on the third day of the month. Now reckon the days between the 14th day of the first month, the day of the passover, and the 3d day of the third month, and you have 17 days of the first month, 30 of the second, and 3 of the third— 50 in all. The Law in the Ark of the Testimony represents holiness in the Lord's body, by whose resurrection is promised to us the future rest; for our receiving of which, love is breathed into us by the Holy Spirit. But the Spirit had not then been given, for Jesus had not yet been glorified. Hence that prophetic song, “Arise, O Lord, into Your rest, You and the ark of Your strength” [holiness, LXX.]. Where there is rest, there is holiness. Wherefore we have now received a pledge of it, that we may love and desire it. For to the rest belonging to the other life, whereunto we are brought by that transition from this life of which the passover is a symbol, all are now invited in the name of the Father, the Son, and the Holy Spirit.
Source: Letters (New Advent)