14 But He came down to clothe the Apostles with power, and to baptize them; for the Lord says, you shall be baptized with the Holy Ghost not many days hence. This grace was not in part, but His power was in full perfection; for as he who plunges into the waters and is baptized is encompassed on all sides by the waters, so were they also baptized completely by the Holy Ghost. The water however flows round the outside only, but the Spirit baptizes also the soul within, and that completely. And wherefore do you wonder? Take an example from matter; poor indeed and common, yet useful for the simpler sort. If the fire passing in through the mass of the iron makes the whole of it fire, so that what was cold becomes burning and what was black is made bright—if fire which is a body thus penetrates and works without hindrance in iron which is also a body, why wonder that the Holy Ghost enters into the very inmost recesses of the soul?
Source: Catechetical Lectures (New Advent)