“Although in ways known to himself God can lead those who, through no fault of their own, are ignorant of the Gospel, to that faith without which it is impossible to please him, the Church still has the obligation and also the sacred right to evangelize all men.”
Saint John Paul II wrote this when he mentioned the Church’s teaching on those who never received the message of the Gospel, hence “through no fault of their own” were “ignorant of the Gospel” and could not possibly have made the conscious decision to not become fully integrated into the Church (cf. Catechism of the Catholic Church, no. 847). Such individuals may also be counted among the elect, though I would not say this means they go straight to Heaven but first undergo additional purgation in Limbo or Purgatory. Saint Thomas Aquinas spoke about invincible ignorance.
“For this reason such like ignorance, not being voluntary, since it is not in our power to be rid of it, is not a sin: wherefore it is evident that no invincible ignorance is a sin.” (Summa Theologica I-II.76.2)
Blessed Pius IX himself admitted certain circumstances in which one who is not a member of the Church can be saved when he said, “There are, of course, those who are struggling with invincible ignorance about our most holy religion” and “[s]incerely observing the natural law and its precepts inscribed by God on all hearts and ready to obey God, they live honest lives and are able to attain eternal life by the efficacious virtue of divine light and grace” (cf. Quanto Conficiamur Moerore).
I once saw a YouTube video by a Sedevacantist who argued Pius IX’s statement does not count because he is not a saint; this falls flat on itself for many reasons: (1) who is to say he won’t be canonized in the future as many saints are not canonized until centuries after their deaths, (2) there is no Church teaching that a theologian must be a canonized saint to have authority in teaching, (3) he is beatified which means he is recognized to be among the elect and on the way to canonization, (4) many saints have no been canonized and many are unnamed, (4) he is the pope declared the Immaculate Conception dogma, so those who reject his teaching on invincible ignorance might as well reject his teaching on the Immaculate Conception of Mary.
Here is a Sevacantist site quoting in favor of this subject: http://www.onetruecatholicfaith.com/Roman-Catholic-Articles.php?id=308&title=St.+Thomas+rejected+%22Invincible+Ignorance%22&category=Catholic+Articles&page=2
The Slaves of the Immaculate Heart of Mary, the same order founded by Fr. Leonard Feeney, has a great article on invincible ignorance in the case of Amerindians of the pre-Columbian period.