What??
If one is invincibly ignorant, by what means is grace forfeited, since full knowledge is necessary for the grave sin by which the grace is lost?
Conversely, if the grace is not forfeited, how would such a one be damned?
Those who do not believe in Christ, in His Church, who do not have the Catholic faith, sin. The sin of unbelief.
John 16:8-9 And when he is come, he will convict the world of sin, and of justice, and of judgment. 9
Of sin: because they believed not in me.