Fr. Regis Scanlon has recently pointed out in his take on Vatican II that Ultra traditionalists take great offense at this Vatican II statement:
Fr. Scanlon writes: "Traditionalists counter with Boniface VII’s statement in his bull, Unam Sanctam, that “outside the Church there is no salvation” (Nov. 18, 1302, Denz. No. 468, 30th edition).Those who through no fault of their own do not know the Gospel of Christ and His Church, but who nonetheless seek God with a sincere heart and, moved by divine grace, try in their actions to do his will as they know it through the dictates of their conscience—those too may achieve eternal salvation. Nor shall divine providence deny the assistance necessary for salvation to those, who through no fault of their own, have not yet arrived at the explicit knowledge of God, and who, not without grace, strive to lead a good life (Lumen Gentium, no. 16).
Of course what Boniface said is true. But how God introduces His Church to each individual cannot be fully gleaned by any human being. Vatican II merely acknowledged that God is free to distribute his grace where He wills—and that He has access to each human heart in ways that we are not privy to". This entire debate has been taken up by Ralph Martin here- a commendable book I am working through now!