113. But perhaps some one will say that the Seraph said to Isaiah: “Behold, this hath touched thy lips, and shall take away thine iniquities, and purge away thy sins.” Is. vi. 7. 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.” Zech. iii. 2, 3.