11 Let then such as these, the Apostles of Christ, the preachers of the Gospel, who “salute not by the way,” that is, who do not seek or do any other thing, but who in genuine charity preach the Gospel, let them come into the house, and say, “Peace to this house.” They speak not with the mouth only; they pour out that of which they are full; they preach peace, and they have peace. They are not as those of whom it was said, “Peace, Peace, and there is no peace.” What is, “Peace, Peace, and there is no peace”?
They preach it, but they have it not; they praise it and they love it not; they say, and do not. But yet do you receive the peace, “whether by occasion or in truth Christ be preached.” Whoso then is full of peace, and salutes, saying, “Peace to this house, if the son of peace be there, his peace shall rest upon him; if not,” for perhaps there is no one of peace there, yet he who saluted has lost nothing, “it shall return,” says he, “to you again.” It shall return to you, though it never departed from you.
For this He would mean to say, It profits you that you have declared it, it has not profited him at all who has not received it; you have not lost your reward, because he has remained empty; it is rendered you for your good will, it is rendered you for the charity which you have bestowed, He will render it to you who has given you assurance of it by that Angelic voice, “Peace on earth to men of good will.”
Source: Sermons on Selected Lessons of the New Testament (New Advent)