From the CCC:
"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 achieve eternal salvation" (CCC 847)
"If...the ignorance is invincible, or the moral subject is not responsible for his erroneous judgment, the evil committed by the person cannot be imputed to him. It remains no less an evil, a privation, a disorder. One must therefore work to correct the errors of moral conscience" (CCC 1793).
"However, no one is presumed to be ignorant of the principles of moral law since these are written on the heart of every man" (CCC 1860).
Syllabus of errors(1864) of Pope Pius IX
Condemned Proposition #17: "Good hope at least is to be entertained of the eternal salvation of all those who are not at all in the true Church of Christ. -- Encyclical ‘Quanto Conficiamur,’ Aug. 10, 1863, etc.”Pope Pius IX, Quanto Conficiamur Moerore, 1863, #7: "Here, too, our beloved sons and venerable brothers, it is again necessary to mention and censure a very grave error entrapping some Catholics who believe that it is possible to arrive at eternal salvation although living in error and alienated from the true faith and Catholic unity. Such belief is certainly opposed to Catholic teaching.