From “Sermon 75” (ca. 450)

Today’s solemnity is to be honored as one of the chief feasts, and there is no doubt that great respect is due to this day, which the Holy Spirit has hallowed by the miracle of his most excellent gift. For from the day on which the Lord ascended above all heavenly heights to sit down at God the Father’s right hand, this is the tenth, and the fiftieth from his resurrection. This day reveals mysteries both new and old, for grace was announced before through the law and the law is fulfilled through grace.

When the Hebrew nation was released from the Egyptians, on the fiftieth day after the sacrificing of the lamb, the law was given on Mount Sinai. Likewise, after the suffering of Christ, wherein the true Lamb of God was slain, on the fiftieth day from his resurrection, the Holy Spirit came down upon the apostles and the multitude of believers. This was so that earnest Christians could see that that the beginnings of the Old Testament were preparatory to the beginnings of the Gospel, and that the second covenant was founded by the same Spirit that had instituted the first…

Let us with one mind be incited to pay reverence to Whitsuntide, rejoicing in honor of the Holy Spirit, by whom the whole catholic Church is sanctified, and every rational soul quickened. He is the inspirer of the faith, the teacher of knowledge, the fount of love, the seal of chastity, and the cause of all power. Let the minds of the faithful rejoice, that throughout the world one God, Father, Son, and Holy Ghost, is praised by the confession of all tongues, and that that sign of his presence, which appeared in the likeness of fire, is still perpetuated in his work and gift. For the Spirit of truth himself makes the house of his glory shine with the brightness of his light, and will have nothing dark nor lukewarm in his temple.

St. Leo the Great (ca. 400-461) was a Roman cleric and theologian. He served as a diplomat for the papal court and became Bishop of Rome in 440, exercising pastoral care during the depredations of the Huns and the Vandals. Nearly 100 of his sermons are preserved, many of them associated with the primary feasts and fasts of the Church Year. His feast day is November 10. The text is adapted for contemporary readers.