And, again from St. Paul: When Gentiles who have not the law do by nature what the law requires, they are a law to themselves, even though they do not have the law. They show that what the law requires is written on their hearts, while their conscience also bears witness and their conflicting thoughts accuse or perhaps excuse them on that day when, according to my gospel, God judges the secrets of men by Christ Jesus. (Rom 2:14-16)
Pope Pius IX, December 9th, 1854 said:
And...We must hold as of the faith, that out of the Apostolic Roman Church there is no salvation; that she is the only ark of safety, and whosoever is not in her perishes in the deluge; we must also, on the other hand, recognize with certainty that those who are invincible in ignorance of the true religion are not guilty for this in the eyes of the Lord. And who would presume to mark out the limits of this ignorance according to the character and diversity of peoples, countries, minds and the rest?
in Quanto conficiamur moerore 10 August, 1863:
It is known to us and to you that those who are in invincible ignorance of our most holy religion, but who observe carefully the natural law, and the precepts graven by God upon the hearts of all men, and who being disposed to obey God lead an honest and upright life, may, aided by the light of divine grace, attain to eternal life; for God who sees clearly, searches and knows the heart, the disposition, the thoughts and intentions of each, in His supreme mercy and goodness by no means permits that anyone suffer eternal punishment, who has not of his own free will fallen into sin.
The problem is, of course, that without the sacraments, and especially Baptism, Eucharist and Confession, it is almost impossible not to fall into mortal sin with one's free will. It is hard to imagine anyone in today's world maintaining innocence, without mortal sin, being saved outside the Church, which gives us opporunities for grace over and over and over.
Sadly, God will actually stop giving grace if persons decide consistently to choose mortal sin over His Truth, either as given to all in natural law, as codified in the Ten Commandments, (which is the natural law "written in stone" ), and the holiness called for in the Beatitudes. Remember, only the perfect see God, so in addition, one must submit to purgation.
I am not sure there is such a thing as invincible ignorance for adults in this day and age of information glut.
Yes, God gives sufficient grace to all men and women, but to those who turn towards Him, He gives more and more grace.
And, contrary to some clergy and lay people, a misinformed or underdeveloped conscience in a Catholic is not invincible ignorance.
Christ is clear on what one needs for salvation as seen in these passages: Mark 16:16, Luke 13:3 John 6:54.
And from the CCC: Basing itself on Scripture and Tradition, the Council teaches that the Church, a pilgrim now on earth, is necessary for salvation: the one Christ is the mediator and the way of salvation; he is present to us in his body which is the Church. He himself explicitly asserted the necessity of faith and baptism, and thereby affirmed at the same time the necessity of the Church which men enter through baptism as through a door. Hence they could not be saved who, knowing that the Catholic Church was founded as necessary by God through Christ, would refuse either to enter it or to remain in it. (CCC 846)