Many Doctors, Fathers and Saints have held that, if necessary, God would rather send an Angel to instruct souls in the Catholic Faith, or provide this to them by some other such extraordinary means, than allow them to perish in ignorance of the essential mysteries of the Faith necessary by a necessity of means.
Tradition (for e.g. the Athanasian Creed) says whoever wishes to be saved must hold the Catholic Faith, principally the Trinity and Incarnation, which excludes all pagans and Jews, and whoever does not hold the whole Catholic Faith firmly and faithfully "without doubt, he will perish forever", which excludes all heretics and schismatics. In this sense, and in this understanding, the Faith has been always and everywhere held, as the following quotes will make evident.
Msgr. Fenton informs us,
Most theologians teach that the minimum explicit content of supernatural and salvific faith includes, not only the truths of God’s existence and of His action as the Rewarder of good and the Punisher of evil, but also the mysteries of the Blessed Trinity and the Incarnation.
I've never read an explanation of the following statement from those who hold the minority position.
"We declare that a great number of those who are condemned to eternal punishment suffer that everlasting calamity because of ignorance of those mysteries of Faith which must be known and believed in order to be numbered among the elect."
A mystery of Faith is defined by the First Vatican Council as the proper object of supernatural faith, as a truth to which natural reason cannot attain, but which must be revealed by God to be known. Obviously, then, since St. Pius X tells us there are some mysteries of Faith that must be known to be numbered among the elect, these mysteries can only be the Trinity and Incarnation. And St. Pius X does not say merely "believed", so that someone could argue that this could be explained by implicit belief, but "known" and there is no way around "known".
Even before Pope St. Pius X, Fr. Mueller had written,
"Some theologians hold that the belief of the two other articles - the Incarnation of the Son of God, and the Trinity of Persons - is strictly commanded but not necessary, as a means without which salvation is impossible; so that a person inculpably ignorant of them may be saved. But according to the more common and truer opinion, the explicit belief of these articles is necessary as a means without which no adult can be saved."
The discredited minority opinion never found favor with the great Doctors, and for good reason. As is clear in the quotes below, they rejected such a view, argued against and refuted it, God would provide all that was necessary for every infidel to be converted to the Faith, provided he for his part did not refuse the series of actual graces leading up to this end that God gave him.
Still we answer the Semipelagians, and say, that infidels who arrive at the use of reason, and are not converted to the Faith, cannot be excused, because though they do not receive sufficient proximate grace, still they are not deprived of remote grace, as a means of becoming converted. But what is this remote grace? St. Thomas explains it, when he says, that if anyone was brought up in the wilds, or even among brute beasts, and if he followed the law of natural reason, to desire what is good, and to avoid what is wicked, we should certainly believe either that God, by an internal inspiration, would reveal to him what he should believe, or would send someone to preach the Faith to him, as he sent Peter to Cornelius. Thus, then, according to the Angelic Doctor, God, at least remotely, gives to infidels, who have the use of reason, sufficient grace to obtain salvation, and this grace consists in a certain instruction of the mind, and in a movement of the will, to observe the natural law; and if the infidel cooperates with this movement, observing the precepts of the law of nature, and abstaining from grievous sins, he will certainly receive, through the merits of Jesus Christ, the grace proximately sufficient to embrace the Faith, and save his soul.”
And St. Thomas likewise, after teaching that all must make an act of the Catholic Faith, in explicitly at least the Trinity and Incarnation to be saved, answers the objection below. Infidels who die as infidels, therefore, and likewise, heretics who die as heretics, are lost. To be saved, to St. Thomas and the Doctors, as to St. Athanasius and the Fathers, and to St. Pius X and the Popes, one must convert and be Catholic before one's death.
Objection: “It is possible that someone may be brought up in the forest, or among wolves; such a man cannot explicitly know anything about the faith."
St. Thomas replies: "I answer that ... it is the characteristic of Divine Providence to provide every man with what is necessary for salvation provided on his part there is no hindrance. In the case of a man who seeks good and shuns evil, by the leading of natural reason, God would either reveal to him through internal inspiration what had to be believed, or would send some preacher of the Faith to him.”