Lumen Gentium (excerpt)
DOGMATIC CONSTITUTION ON THE CHURCH
Vatican Council II
November 21, 1964
16. Finally, those who have not yet received the Gospel are related to the People of God in various ways.18 There is, first, that people to whom the covenants and promises were made, and from whom Christ was born according to the flesh (cf. Rom. 9:45): in view of the divine choice, they are a people most dear for the sake of the fathers, for the gifts of God are without repentance (cf. Rom. 11:2929). But the plan of salvation also includes those who acknowledge the Creator, in the first place amongst whom are the Moslems: these profess to hold the faith of Abraham, and together with us they adore the one, merciful God, mankind’s judge on the last day. Nor is God remote from those who in shadows and images seek the unknown God, since he gives to all men life and breath and all things (cf. Acts 17:2528), and since the Savior wills everyone to be saved (cf. 1 Tim. 2:4). Those who, through no fault of their own, do not know the Gospel of Christ or his Church, but who nevertheless seek God with a sincere heart, and, moved by grace, try in their actions to do his will as they know it through the dictates of their conscience—those too may attain eternal salvation.19 Nor shall divine providence deny the assistance necessary for salvation to those who, without any fault of theirs, have not yet arrived at an explicit knowledge of God, and who, not without grace, strive to lead a good life. Whatever good or truth is found amongst them is considered by the Church to be a preparation for the Gospel20 and given by him who enlightens all men that they may at length have life. But very often, deceived by the Evil One, people have lost their way in their thinking, have exchanged the truth of God for a lie and served the creature rather than the Creator (cf. Rom. 1:21 and 25). Or else, living and dying in this world without God, they are exposed to ultimate despair. Hence to procure the glory of God and the salvation of all these people, the Church, mindful of the Lord’s command, "preach the Gospel to every creature" (Mk. 16:16) takes great care to encourage the missions.
18. See St. Thomas, Summa Theol. III, q. 8, a. 3, ad. 1.
19. See Epist. of the Sacred Cong. of the Holy Office to the Archbishop of Boston: Denz. 3869-72.
20. See Eusebius of Caesarea, Praeparatio Evangelica, I,1: PG 21,28 AB.