From "A Treatise Against the Heresies" by St Irenaeus of Lyon (he says it so much better than I can)
When the Lord told his disciples to go and teach all nations and baptise them in the name of the Father and of the Son and of the Holy Spirit, he conferred on them the power of giving men new life in God. He had promised through the prophets that in these last days he would pour out his Spirit on his servants and handmaids, and that they would prophesy. So when the Son of God became the Son of Man, the Spirit also descended upon him, becoming accustomed in this way to dwelling with the human race, to living in men and to inhabiting God’s creation. The Spirit accomplished the Father’s will in men who had grown old in sin, and gave them new life in Christ.
Luke says that the Spirit came down on the disciples at Pentecost, after the Lord’s ascension, with power to open the gates of life to all nations and to make known to them the new covenant. So it was that men of every language joined in singing one song of praise to God, and scattered tribes, restored to unity by the Spirit, were offered to the Father as the first-fruits of all the nations.
This was why the Lord had promised to send the Advocate: he was to prepare us as an offering to God. Like dry flour, which cannot become one lump of dough, one loaf of bread, without moisture, we who are many could not become one in Christ Jesus without the water that comes down from heaven. And like parched ground, which yields no harvest unless it receives moisture, we who were once like a water less tree could never have lived and borne fruit without this abundant rainfall from above. Through the baptism that liberates us from change and decay we have become one in body; through the Spirit we have become one in soul.
The Spirit of wisdom and understanding, the Spirit of counsel and strength, the Spirit of knowledge and the fear of God came down upon the Lord, and the Lord in turn gave this Spirit to his Church, sending the Advocate from heaven into all the world into which, according to his own words, the devil too had been cast down like lightning. If we are not to be scorched and made unfruitful, we need the dew of God. Since we have our accuser, we need an advocate as well. And so the Lord in his pity for man, who had fallen into the hands of brigands, having himself bound up his wounds and left for his care two coins bearing the royal image, entrusted him to the Holy Spirit. Now, through the Spirit, the image and inscription of the Father and the Son have been given to us, and it is our duty to use the coin committed to our charge and make it yield a rich profit for the Lord.
you sanctify your Church in every race and nation
by the mystery we celebrate on this day.
Pour out the gifts of the Holy Spirit on all mankind,
and fulfill now in the hearts of your faithful
what you accomplished when the Gospel
was first preached on earth.
was first preached on earth.
Through our Lord Jesus Christ, your Son,
who lives and reigns with you and the Holy Spirit,
one God, for ever and ever. Amen.