These things I say, because I desire to abase the mind of my hearer in humility. But let your very humility have its confidence. For I, a sinner, maintain a most certain hope that through the grace of our Almighty Creator and Redeemer, our God and Lord Jesus Christ, your sins are already remitted, and you are chosen for this purpose, that those of others may be remitted through you. Nor will you have sorrow for any guilt in the future, while you strive to cause joy in heaven for the conversion of many. Truly the same our Maker and Redeemer, speaking of the repentance of men, says, Verily I say unto you there will be joy in heaven over one sinner that repents, more than over ninety and nine just persons, which need no repentance. And if for one penitent there is great joy in heaven, of what kind may we believe the joy to be for so large a people, converted from its error, which, coming to faith, has condemned by penitence the evil things it did. In this joy, then, of heaven and the angels let us repeat the very words of the angels with which we began: let us say therefore, let us all say, Glory to God in the highest, and on earth peace to men of good will.
Source: Register of Letters (New Advent)