I go to Him that sent Me; but because I have spoken these things to you, sorrow hath filled your heart, alleluia. (Magnificat Antiphon) “For if I go not, the Paraclete will not come to you: but if I go, I will send Him to you.” (Gospel)
Today's liturgy exalts the justice of God which is shown forth by our Lord's triumph, and by the sending of the Holy Ghost. "The right hand of the Lord hath wrought strength" in raising Christ from the dead (Alleluia) and in causing Him to go into heaven on the day of His ascension. It is expedient for us that Jesus should leave the earth, for from heaven He will send to His Church the Spirit of truth (Gospel), that best gift which comes from the Father of lights (Epistle).
The Holy Ghost will teach us all truth (Gospel, Offertory, Secret), He will "show us" whatever our Lord tells Him, and in receiving this word of life we shall be saved (Epistle). The Holy Ghost will reveal to us the wonderful things which God hath wrought for His Son and this testimony of signal justice done to our Lord, will console our souls and uphold us in the midst of persecutions.
And since, according to St James, the trying of our faith worketh patience which drives away inconstancy and makes perfect our good deeds, so let us imitate the patience of our God and Father, in whom "there is no change or shadow of alteration" (Epistle), and then our hearts will be "set where true joys are to be found" (Collect).
On the other hand, the Holy Ghost will also convince Satan and the world of the sin they committed in delivering Jesus Christ to death (Gospel, Communion), and in continuing to persecute Him in His Church.
Source: Dom Gaspar Lefebvre, OSB, 1945, adapted and abridged.