"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." St. Irenaeus, an early Greek bishop, meditates on Pentecost and the role of the Holy Spirit in salvation history.
St. Luke says that the Spirit came down upon the disciples on 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 nations. This is 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 loaf of bread without moisture, we who are many could not become one in Jesus Christ 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 waterless tree, could have never lived and borne fruit without this abundant rainfall from above.
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.
Since we have our accuser, we need an Advocate as well. And so, the Lord, in his pity for man, entrusted him to the Holy Spirit.