8 And John also, who had been filled with the Holy Ghost from his mother's womb, was for this cause sanctified, that he might baptize the Lord; not giving the Spirit himself, but preaching glad tidings of Him who gives the Spirit. For he says, I indeed baptize you with water unto repentance, but He that comes after me, and the rest; He shall baptize you with the Holy Ghost and with fire. But wherefore with fire? Because the descent of the Holy Ghost was in fiery tongues; concerning which the Lord says joyfully, I have come to send fire on the earth; and what will I, if it be already kindled?
Source: Catechetical Lectures (New Advent)