Our Lord laid the foundations of His Church during His public life, and after His resurrection He gave it the powers necessary for its mission.
It was by the Holy Ghost that the apostles were to be trained and endued with strength from on High (Gospel). At Pentecost we celebrate the first manifestation of the Holy Ghost among our Lord's disciples and the foundation of the Church itself.
We read in the Gospel that our Lord foretold the coming of the Paraclete to His disciples and the Epistle shows us the realization of that promise. It was at the third hour of the day (Terce, nine o'clock a. m.) that the Spirit of God descended upon the Cenacle and a mighty wind which blew suddenly upon the house, together with the appearance of tongues of fire within, were the wonderful tokens of His coming. Taught by the "light of Thy Holy Ghost" (Collect), and filled by the gifts of the same Spirit poured out upon them (Sequence), the apostles become new men to go forth and renew the whole world (Introit). It is at High Mass, at the third hour, that we also receive the Holy Ghost whom our Lord "going up above all the heavens, on this day sent down ... on the children of adoption" (Preface); for each of the mysteries of the cycle brings forth its fruits of grace in our souls on the day which the Church keeps as its anniversary.
Almighty God, in making known to us that His one divine Nature is possessed by three distinct Persons, reveals to us something of His own interior life. Let us, with the Church, say to the Holy Ghost, "Come, O Holy Ghost, and fill the hearts of Thy faithful, and kindle in them the fire of Thy love" (Alleluia).
Source: Dom Gaspar Lefebvre, OSB, 1945, adapted and abridged.