Justin mentions the subject in his First Apology when he is interpreting Jacob's Blessing in Gen. xlix. The passage is given in full above on p.7.
"The blood of the grape," he says, "signifies that
This interpretation of the words "Holy Spirit" in Matt. i 20 and Luke i.35 is all the more striking because it follows immediately upon the reference to the "prophetic Spirit," whose function it was to announce the birth from the Virgin beforehand. No further comment is necessary here on this passage; but it may be worth while to note that the belief that the Word was Himself the agent of His own Incarnation finds its natural place side by side with the belief that it is through His direct agency, and not through that of the Holy Spirit, that the bread and wine of the Eucharist are made the Body and Blood of the Incarnate Word: see the well-known passage in Ap.1, 66, where however Justin's intricate constructions make the exact meaning of his words difficult to determine.
While "the prophetic Spirit" is thus expressly excluded from, the part in the mystery of the Incarnation which a later interpretation of the words of the Gospels assigned to Him, it is to be noted that Justin makes much of His descent upon Christ at the Baptism. In Dial.87 the Jew Trypho is made to quote Isa. xi.2-3: "The Spirit of the Lord shall rest upon him, the Spirit of wisdom and understanding," etc. Conceding that it is Christ on whom the Spirit is to rest in His sevenfold power, Trypho proceeds to ask how, if Christ be God, He should be in need of this gift. Justin's answer is that He is in no such need that, when the prophet says that the Spirit shall "rest" upon Him, he means that He will go no further, that He will have reached a termination, so far as His prophetic work among the Jewish people is concerned. This, he says, you yourselves see to be true: you have had no prophet since. The gifts enumerated were divided among your prophets, some had one, some had another. But they all met on Christ. "When He was come, the Spirit rested, paused" (anepausato oun, toutestin epausato, elthontos ekeinou).  A new era then began, in which Christ "having received gifts," as was prophesied, "gives them, from the grace of the power of that Spirit, to those who believe on Him, according as He knows each to be worthy." Today "you can see among us both women and men who have gifts of grace (charisimata) from the Spirit of God" (c.88). In an earlier chapter he had said (c.82: cf. also c.39): "Among us at the present time there are gifts of prophecy (prophetic charismata); "and he had just before referred to the prophecies of St John's Apocalypse.
While Justin thus recognizes the existence of special gifts of the Holy Spirit in the Church, he does not expressly connect Him with the ordinary graces of the Christian life. Even when he is dealing with the interpretation of the prophecies inspired by the Holy Spirit, he does not say, as later writers do, that we need the enlightenment of the same Holy Spirit to explain their meaning: he says, again and again, that we need "the grace of God" for this purpose. And just as he stops short of saying that this "grace" is, or proceeds from, the Holy Spirit, so also he stops short of saying that "the living water" given by Christ, the true Rock, is the Holy Spirit (Dial.114).
We pass now from Justin's teaching about the Holy Spirit to that of Irenæus in the Demonstration, to which we shall add illustrations taken from his larger work Against Heresies. It will be convenient at first at any rate to consider it under the same headings as before.