That the Spirit forgives sin is common to Him with the Father and the Son, but not with the Angels.
112. Tell me, then, whoever you are who deny the Godhead of the Holy Spirit. The Spirit could not be liable to sin, Who rather forgives sin. Does an Angel forgive? Does an Archangel? Certainly not, but the Father alone, the Son alone, and the Holy Spirit alone. Now no one is unable to avoid that which he has power to forgive.
113. But perhaps some one will say that the Seraph said to Isaiah: “Behold, this has touched your lips, and shall take away your iniquities, and purge away your sins.” Shall take away, he says, and shall purge, not I will take away, but that fire from the altar of God, that is, the grace of the Spirit. For what else can we piously understand to be on the altar of God but the grace of the Spirit? Certainly not the wood of the forests, nor the soot and coals. Or what is so in accordance with piety as to understand according to the mystery that it was revealed by the mouth of Isaiah that all men should be cleansed by the passion of Christ, Who as a coal according to the flesh burnt up our sins, as you read in Zechariah: “Is not this a brand cast forth from the fire? And that was Joshua clothed in filthy garments.”
114. Lastly, that we may know that this mystery of the common redemption was most clearly revealed by the prophets, you have also in this place: “Lo, it has taken away your sins;” not that Christ put aside His sins Who did no sin, but that in the flesh of Christ the whole human race should be loosed from their sins.
115. But even if the Seraph had taken away sin, it would have been as one of the ministers of God appointed to this mystery. For thus said Isaiah: “For one of the Seraphim was sent to me.”
Source: On the Holy Spirit (New Advent)