Good-will exists especially in the Church, and nourishes kindred virtues.
170. Good-will expands in the body of the Church, by fellowship in faith, by the bond of baptism, by kinship through grace received, by communion in the mysteries. For all these bonds claim for themselves the name of intimacy, the reverence of children, the authority and religious care of parents, the relationship of brothers. Therefore the bonds of grace clearly point to an increase of good-will.
171. The desire to attain to like virtues also stands one in good stead; just as again good-will brings about a likeness in character. For Jonathan the king's son imitated the gentleness of holy David, because he loved him. Wherefore those words: “With the holy you shall be holy,” seem not only to be concerned with our ordinary intercourse, but also to have some connection with good-will. The sons of Noah indeed dwelt together, and yet their characters were not at all alike. Esau and Jacob also dwelt together in their father's house, but were very unlike. There was, however, no good-will between them to make the one prefer the other to himself, but rather a rivalry as to which should first get the blessing. Since one was so hard, and the other gentle, good-will could not exist as between such different characters and conflicting desires. Add to this the fact that holy Jacob could not prefer the unworthy in son of his father's house to virtue.
172. But nothing is so harmonious as justice and impartiality. For this, as the comrade and ally of good-will, makes us love those whom we think to be like ourselves. Again, good-will contains also in itself fortitude. For when friendship springs from the fount of good-will it does not hesitate to endure the great dangers of life for a friend. “If evils come to me through him,” it says, “I will bear them.”
Source: On the Duties of the Clergy (New Advent)