Defending the Faith: Instructing the Ignorant

Another source of confusion about allies and enemies is a misunderstanding of the Church's teaching on invincible ignorance and non-Catholics. Some apologists ... think that once Catholicism has been explained to the intelligent Protestant, for instance, he can no longer be invincibly ignorant of it. If he remains a Protestant, he cannot be saved. His remaining a Protestant in such a situation only proves to some apologists that he is not a good man. He is, they think, an enemy, not an ally, of truth.

We should be clear: those who hold this view do not deny that non-Catholics can be saved. They simply think that reasonable non-Catholics who have heard the case for the Catholic Church can no longer be invincibly ignorant. If such a non-Catholic fails to convert, it must be because he is, in his heart of hearts, stubbornly opposing the truth, and, therefore, stubbornly opposing God himself...

The trouble with this scenario is that it does not accurately represent Catholic teaching about ignorance and culpability. Invincible ignorance does not mean one is merely ignorant, through no fault of his own, of what the Catholic Church teaches; it means one is ignorant of its truth -- of the fact that it is true -- through no fault of his own ... after all is said and done, after the Catholic apologist has framed the best arguments he can and after the Protestant has investigated Catholicism honestly and to the best of his ability, the Protestant may still (erroneously and mysteriously, perhaps) think that Catholicism is false or less than completely true. If the Protestant were to become Catholic under those circumstances, he would be embracing what he thinks is untrue. And that would amount to sin, not salvation for him.

How Not to Share Your Faith:
The Seven Deadly Sins of Apologetics
by Mark Brumley

Again we see the great tribute paid to conscience. If someone is following theirs truly even if they don't agree with you, then they have the right to be wrong (as, indeed, we should have in their eyes).