This sermon exists not only in the Ambrosian codex, but also in the Turin one, in which it is held as homily 21 by number. It is manifest, brethren, and clear to all, how great is the care of God the almighty Father for all mortals; for whom it was too little to have delivered His only Son to death for us, unless He had also poured out into the hearts of His faithful the fullness of gifts, as was said by the Lord through the Prophet: 'In the last days I will pour out of My Spirit upon My menservants and My maidservants, and I will give wonders in heaven above, and signs on earth beneath. The sun shall be turned into darkness, and the moon into blood' (Joel 2).
The Lord indeed gives wonders in heaven, when at the nativity of Christ a new star shines forth before the eyes of the Chaldeans, and at His Passion the midday sun sinks, darkened by a sudden gloom. He gives signs on earth, when the earth itself trembles with unwonted heavings, when rocks are rent and split apart into pieces, when even the tombs pour back to the light the bodies of the departed. 'The sun,' he says, 'shall be turned into darkness, and the moon into blood.' What else is it, dearly beloved—if we contemplate spiritual things—for the sun to be turned into darkness, but when Christ passes into death; or for the moon to be turned into blood, but when the holy Church is stained with blood in various sufferings for the confession of her Christ?
Christ is delivered to death, not that He should remain in the corruption of the tomb, but that after the darkness of hell He might shine forth more brightly. The Church is harried by the hands of the bloodthirsty, not that she should fail amid torments and punishments, but that through the manifold titles of victories she might be adorned in all her beauty with the blood of the triumphant martyrs. But all these things one and the same grace of the concordant Christ and Spirit works and accomplishes. Christ, indeed, who kindles the breasts of His servants unto every desire of suffering by the hope of eternal recompense. The Spirit likewise, by the vigor of divine power, confirms the hearts of believers in contempt of death.
And therefore, after the Lord returned victorious to the Father, and received His own into the kingdom, the paternal grace bestowing it with Him, He gave His apostles the Paraclete, whom He had promised, so that the Spirit, coming from above, might guard the faith which He Himself had planted in them. The Spirit therefore descended upon the apostles, Christ promising it, so that it might appear that there is one indissoluble will of them in all things. How fittingly, moreover, does the Divinity itself insinuate itself into human perceptions by alternate testimony, when Christ foretells that the Spirit will come from heaven, and the Spirit attests that Christ reigns in heaven! But who could doubt that the voice is most true, both of Him who goes to heaven, and of Him who comes from heaven?
So the apostles were filled today, as it was read, with the Holy Spirit, and began to speak with various tongues, as the Spirit gave them to utter (Acts 2). The Spirit filled those, indeed, whose hearts the blood and doctrine of Christ had purified for His coming. And this was the first benefit of the coming Spirit: that these men, who had been ordained for the salvation of all nations, might speak in the speech of all. And the grace of the Paraclete so filled both the tongue and the breast of the apostles, that neither was power lacking to their words, nor words to their power. Nor indeed is there anything else in divine and human affairs that so makes a man glorious as the perfection of holy speech and the fullness of heavenly fortitude.
The apostles therefore spoke through one Spirit with various tongues, that the diversity of the nations might converge into the one faith of Christ. Whence there is no doubt that this Holy Spirit is to be confessed as God with the Father and the Son: for who else could do these things, save God, that a Roman man, and a Parthian too, and a Mede, and a Phrygian, and every barbarian, a stranger in Judea, should hear his own tongue, and a man of Galilee should speak the secrets of heaven to barbarian ears, not only with the elegance of Greek and Roman speech, but even with a shrill murmur and a groaning effort?
For once, when human presumption, striving to go to heaven, built itself a tower, and its stubborn effort had no effect, the fellowship of one speech was divided by the sudden infusion of a variety of tongues, so that, while one did not understand the other conversing, all that fabric of impiety was straightway dissolved. But now, when God prepares for man an ascent to heaven, to the heralds is given such expertise in the grace of all tongues, that the great promise of God might not lie hidden from the man of any nation; as many ages before was said in prophetic speech: 'There are no languages nor tongues in which their voices are not heard' (Psalm 18).
Whence it is manifest, dearly beloved, that the Son of the almighty God, who willed the mystery of eternal salvation to be preached in the truth of all tongues, came as the Redeemer not of the Jewish people alone, but of all men. And therefore, brethren, since the goodness of Christ wills that no mortal should perish, let us be cleansed from all impurity and iniquity, so that, for the help of our life, the most benign Spirit of God may also deign to illumine our hearts, and so forth.