9 Let your feet hasten to the catechisings; receive with earnestness the exorcisms: whether thou be breathed upon or exorcised, the act is to you salvation. Suppose you have gold unwrought and alloyed, mixed with various substances, copper, and tin, and iron, and lead: we seek to have the gold alone; can gold be purified from the foreign substances without fire? Even so without exorcisms the soul cannot be purified; and these exorcisms are divine, having been collected out of the divine Scriptures.
Your face has been veiled, that your mind may henceforward be free, lest the eye by roving make the heart rove also. But when your eyes are veiled, your ears are not hindered from receiving the means of salvation. For in like manner as those who are skilled in the goldsmith's craft throw in their breath upon the fire through certain delicate instruments, and blowing up the gold which is hidden in the crucible stir the flame which surrounds it, and so find what they are seeking; even so when the exorcists inspire terror by the Spirit of God, and set the soul, as it were, on fire in the crucible of the body, the hostile demon flees away, and there abide salvation and the hope of eternal life, and the soul henceforth is cleansed from its sins and has salvation.
Let us then, brethren, abide in hope, and surrender ourselves, and hope, in order that the God of all may see our purpose, and cleanse us from our sins, and impart to us good hopes of our estate, and grant us repentance that brings salvation. God has called, and His call is to you.
Source: Catechetical Lectures (New Advent)