Patristic Wisdom: Looking to Holy Trinity Sunday

Men of Israel, hear these words: Jesus of Nazareth, a Man attested by God to you by miracles, wonders, and signs which God did through Him in your midst, as you yourselves also know… Men and brethren, let me speak freely to you of the patriarch David, that he is both dead and buried, and his tomb is with us to this day. Therefore, being a prophet, and knowing that God had sworn with an oath to him that of the fruit of his body, according to the flesh, He would raise up the Christ to sit on his throne, he, foreseeing this, spoke concerning the resurrection of the Christ, that His soul was not left in Hades, nor did His flesh see corruption. This Jesus God has raised up, of which we are all witnesses. Therefore being exalted to the right hand of God, and having received from the Father the promise of the Holy Spirit, He poured out this which you now see and hear. (Acts 2:22, 29–33)

But the reason why, after His resurrection, He both gave the Holy Spirit, first on earth, and afterwards sent Him from heaven, is in my judgment this: that “love is shed abroad in our hearts,” by that Gift itself, whereby we love God and our neighbors, according to those two commandments, “on which hang all the law and the prophets.” And Jesus Christ, in order to signify this, gave to them the Holy Spirit, once upon earth, on account of the love of our neighbor, and a second time from heaven, on account of the love of God. And if some other reason may perhaps be given for this double gift of the Holy Spirit, at any rate we ought not to doubt that the same Holy Spirit was given when Jesus breathed upon them, of whom He by and by says, “Go, baptize all nations in the name of the Father, and of the Son, and of the Holy Spirit,” where this Trinity is especially commended to us. It is therefore He who was also given from heaven on the day of Pentecost, i.e. ten days after the Lord ascended into heaven. How, therefore, is He not God, who gives the Holy Spirit? Nay, how great a God is He who gives God!

Augustine of Hippo, On the Trinity XV.26.46