In the narration of that event already mentioned, and especially of the sacrifice offered by Nehemiah, is typified the Holy Spirit and Christian baptism. The sacrifice of Moses and Elijah and the history of Noah are also referred to the same.
102. We form the congregation of the Lord. We recognize the propitiation of our Lord God, which our Propitiator wrought in His passion. I think, too, we cannot leave out of sight that fire when we read that the Lord Jesus baptizes with the Holy Spirit and with fire, as John said in his Gospel. Rightly was the sacrifice consumed, for it was for sin. But that fire was a type of the Holy Spirit Who was to come down after the Lord's ascension, and forgive the sins of all, and Who like fire inflames the mind and faithful heart. Wherefore Jeremiah, after receiving the Spirit, says: “It became in my heart as a burning fire flaming in my bones, and I am vile and cannot bear it.” In the Acts of the Apostles, also, when the Holy Spirit descended upon the apostles and those others who were waiting for the Promise of the Father, we read that tongues as of fire were distributed among them. The soul of each one was so uplifted by His influence that they were supposed to be full of new wine, who instead had received the gift of a diversity of tongues.
103. What else can this mean— namely, that fire became water and water called forth fire— but that spiritual grace burns out our sins through fire, and through water cleanses them? For sin is washed away and it is burnt away. Wherefore the Apostle says: “The fire shall try every man's work of what sort it is.” And further on: “If any man's work shall be burned, he shall suffer loss: but he himself shall be saved; yet so as by fire.”
104. This, then, we have stated, so as to prove that sins are burnt out by means of fire. We know now that this is in truth the sacred fire which then, as a type of the future remission of sins, came down upon the sacrifice.
105. This fire is hidden in the time of captivity, during which sin reigns, but in the time of liberty it is brought forth. And though it is changed into the appearance of water, yet it preserves its nature as fire so as to consume the sacrifice. Do not wonder when you read that God the Father said: “I am a consuming fire.” And again: “They have forsaken Me, the fountain of living water.” The Lord Jesus, too, like a fire inflamed the hearts of those who heard Him, and like a fount of waters cooled them. For He Himself said in His Gospel that He came to send fire on the earth and to supply a draught of living waters to those who thirst.
106. In the time of Elijah, also, fire came down when he challenged the prophets of the heathen to light up the altar without fire. When they could not do so, he poured water thrice over his victim, so that the water ran round about the altar; then he cried out and the fire fell from the Lord from heaven and consumed the burnt-offering.
107. You are that victim. Contemplate in silence each single point. The breath of the Holy Spirit descends on you, He seems to burn you when He consumes your sins. The sacrifice which was consumed in the time of Moses was a sacrifice for sin, wherefore Moses said, as is written in the book of the Maccabees: “Because the sacrifice for sin was not to be eaten, it was consumed.” Does it not seem to be consumed for you when in the sacrament of baptism the whole outer man perishes? “Our old man is crucified,” the Apostle exclaims. Herein, as the example of the fathers teaches us, the Egyptian is swallowed up— the Hebrew arises renewed by the Holy Spirit, as he also crossed the Red Sea dryshod— where our fathers were baptized in the cloud and in the sea.
108. In the flood, too, in Noah's time all flesh died, though just Noah was preserved together with his family. Is not a man consumed when all that is mortal is cut off from life? The outer man is destroyed, but the inner is renewed. Not in baptism alone but also in repentance does this destruction of the flesh tend to the growth of the spirit, as we are taught on the Apostle's authority, when holy Paul says: “I have judged as though I were present him that has so done this deed, to deliver him unto Satan for the destruction of the flesh, that the spirit may be saved in the day of our Lord Jesus Christ.”
109. We seem to have made a somewhat lengthy digression for the sake of regarding this wonderful mystery, in desiring to unfold more fully this sacrament which has been revealed to us, and which, indeed, is as full of virtue as it is full of religious awe.
Source: On the Duties of the Clergy (New Advent)