5 There is no need to insist how foreign it is to the virtue of charity, which embraces both God and men, for the members of Christ's Church not to think of those unfortunate souls who live in error outside the Fold. Surely the obligation of charity, which binds us to God, demands not only that we strive to increase by every means within our power the number of those who adore Him "in spirit and in truth" ( John iv, 24) but also that we try to bring under the rule of the gentle Christ as many other men as possible in order that "the profit in his blood" ( Psalms xxix, 10) may be the more and more fruitful and that we may make ourselves the more acceptable to Him to Whom nothing can possibly be more pleasing than that "men should be saved and come to the knowledge of the truth." (I Timothy ii, 4)
Source: Rerum Ecclesiae (Vatican.va)