4 Now therefore concerning this same boldness, let us see what he says. Whence do we understand that charity is perfect? “There is no fear in charity.” Then what say we of him that has begun to fear the day of judgment? If charity in him were perfect, he would not fear. For perfect charity would make perfect righteousness, and he would have nothing to fear: nay rather he would have something to desire; that iniquity may pass away, and God's kingdom come. So then, “there is no fear in charity.”
But in what charity? Not in charity begun: in what then? “But perfect charity,” says he, “casts out fear.” Then let fear make the beginning, because “the fear of the Lord is the beginning of wisdom.” Fear, so to say, prepares a place for charity. But when once charity has begun to inhabit, the fear which prepared the place for it is cast out. For in proportion as this increases, that decreases: and the more this comes to be within, is the fear cast out. Greater charity, less fear; less charity, greater fear.
But if no fear, there is no way for charity to come in. As we see in sewing, the thread is introduced by means of the bristle; the bristle first enters, but except it come out the thread does not come into its place: so fear first occupies the mind, but the fear does not remain there, because it enters only in order to introduce charity. When once there is the sense of security in the mind, what joy have we both in this world and in the world to come! Even in this world, who shall hurt us, being full of charity?
See how the apostle exults concerning this very charity: “Who shall separate us from the charity of Christ? Shall tribulation, or distress, or persecution, or famine, or nakedness, or peril, or sword?” And Peter says: “And who is he that will harm you, if you be followers of that which is good?— There is no fear in love; but perfect love casts out fear: because fear has torment.” The consciousness of sins torments the heart: justification has not yet taken place. There is that in it which itches, which pricks.
Accordingly in the Psalm what says he concerning this same perfection of righteousness? “You have turned for me my mourning into joy: You have put off my sackcloth, and girded me with gladness; to the end that my glory may sing to you, and that I be not pricked.” What is this, “That I be not pricked?” That there be not that which shall goad my conscience. Fear does goad: but fear not: charity enters in, and she heals the wound that fear inflicts. The fear of God so wounds as does the leech's knife; it takes away the rottenness, and seems to make the wound greater.
Behold, when the rottenness was in the body, the wound was less, but perilous: then comes the knife; the wound smarted less than it smarts now while the leech is cutting it. It smarts more while he is operating upon it than it would if it were not operated upon; it smarts more under the healing operation, but only that it may never smart when the healing is effected. Then let fear occupy your heart, that it may bring in charity; let the cicatrice succeed to the leech's knife. He is such an Healer, that the cicatrices do not even appear: only put yourself under His hand.
For if you be without fear, you can not be justified. It is a sentence pronounced by the Scriptures; “For he that is without fear, cannot be justified.” Needs then must fear first enter in, that by it charity may come. Fear is the healing operation: charity, the sound condition. “But he that fears is not made perfect in love.” Why? “Because fear has torment;” just as the cutting of the surgeon's knife has torment.
Source: Homilies on the First Epistle of John (New Advent)