Objection 1. It would seem that peace is not the proper effect of charity. For one cannot have charity without sanctifying grace. But some have peace who have not sanctifying grace, thus heathens sometimes have peace. Therefore peace is not the effect of charity.
Objection 2. Further, if a certain thing is caused by charity, its contrary is not compatible with charity. But dissension, which is contrary to peace, is compatible with charity, for we find that even holy doctors, such as Jerome and Augustine, dissented in some of their opinions. We also read that Paul and Barnabas dissented from one another (Acts 15). Therefore it seems that peace is not the effect of charity.
Objection 3. Further, the same thing is not the proper effect of different things. Now peace is the effect of justice, according to Isaiah 32:17: "And the work of justice shall be peace." Therefore it is not the effect of charity.
On the contrary, It is written (Psalm 118:165): "Much peace have they that love Thy Law."
I answer that, Peace implies a twofold union, as stated above (Article 1). The first is the result of one's own appetites being directed to one object; while the other results from one's own appetite being united with the appetite of another: and each of these unions is effected by charity--the first, in so far as man loves God with his whole heart, by referring all things to Him, so that all his desires tend to one object--the second, in so far as we love our neighbor as ourselves, the result being that we wish to fulfil our neighbor's will as though it were ours: hence it is reckoned a sign of friendship if people "make choice of the same things" (Ethic. ix, 4), and Tully says (De Amicitia) that friends "like and dislike the same things" (Sallust, Catilin.)
Reply to Objection 1. Without sin no one falls from a state of sanctifying grace, for it turns man away from his due end by making him place his end in something undue: so that his appetite does not cleave chiefly to the true final good, but to some apparent good. Hence, without sanctifying grace, peace is not real but merely apparent.
Reply to Objection 2. As the Philosopher says (Ethic. ix, 6) friends need not agree in opinion, but only upon such goods as conduce to life, and especially upon such as are important; because dissension in small matters is scarcely accounted dissension. Hence nothing hinders those who have charity from holding different opinions. Nor is this an obstacle to peace, because opinions concern the intellect, which precedes the appetite that is united by peace. On like manner if there be concord as to goods of importance, dissension with regard to some that are of little account is not contrary to charity: for such a dissension proceeds from a difference of opinion, because one man thinks that the particular good, which is the object of dissension, belongs to the good about which they agree, while the other thinks that it does not. Accordingly such like dissension about very slight matters and about opinions is inconsistent with a state of perfect peace, wherein the truth will be known fully, and every desire fulfilled; but it is not inconsistent with the imperfect peace of the wayfarer.
Reply to Objection 3. Peace is the "work of justice" indirectly, in so far as justice removes the obstacles to peace: but it is the work of charity directly, since charity, according to its very nature, causes peace. For love is "a unitive force" as Dionysius says (Div. Nom. iv): and peace is the union of the appetite's inclinations.
Source: Summa Theologica (New Advent)