A. It is next to impossible for any one in this class to be in invincible ignorance; for, to be in invincible ignorance, three things are necessarily required: first, that a person have a real and sincere desire of knowing the Truth. for if he be cold and indifferent about an affair of so great concern as that of his eternal salvation; if he be careless whether he be in a right way or not; if being enslaved to this present life, he takes no concern about the next, it is manifest, that an ignorance arising from this disposition is a voluntary ignorance , and therefore highly culpable in the sight of God. It will be still worse, if a person be positively unwilling to seek after the Truth, from the fear of worldly inconveniences, and, therefore, industriously avoid every opportunity he may have of knowing it; of such as these the scripture says, "They spend their days in wealth, and in a moment they go down to hell; who have said to God, Depart from us, we desire not the knowledge of Thy ways," (Job xxi. 13) secondly, for one to be in invincible ignorance, it is required, that he be sincerely resolved to embrace the Truth wherever he may find it, and whatever it may cost him. For if he be not fully resolved to follow the will of God, wherever it shall appear to him, in all things necessary to salvation; if, on the contrary, he be so disposed, that he would rather neglect his duty, and hazard his soul, than correct an ill custom, or disoblige his friends, or expose himself for some temporal loss or disadvantage, such a disposition must be highly displeasing to God, and an ignorance arising from it, can never excuse him before his Creator; of this Our Savior says: "He that loveth father or mother, or son or daughter, more than me, is not worthy of me," (Mat. x. 37) The third thing necessary for a person to be in invincible ignorance, is, that he sincerely use his best endeavors to know his duty, and particularly, that he recommend the matter earnestly to Almighty God, and pray for light and direction from Him. For whatever desire he may pretend to have of knowing the Truth, if he does not use the proper means for finding it, it is manifest that his ignorance is not invincible, but voluntary; for ignorance is only there invincible, when a person has a sincere desire to know the Truth, with a full resolution to embrace it, but either has no possible means for knowing it, or after using his best endeavors to know it, yet cannot find it. And, therefore, if a person be deficient in using his endeavors to know his duty, his ignorance is not invincible, it is by his own fault that he does not see it; and, if inattention, indifference, unconcernedness, worldly motives, or unjust prejudices influence his judgement, and suffer it to yield to the bias of a perverse education, he has neither invincible ignorance nor the fear of God. Now it is inconsistent with the goodness and promises of God, that a person brought up in a false religion, but who is disposed as these three conditions require, and use his best endeavors to know the Truth, should be left in an invincible ignorance of it; and if, from his attachment to the world, and to sensual or other selfish objects, he be not so disposed, and neglect to use the proper means for coming at the Truth, then his ignorance is voluntary and culpable, and therefore not invincible.
http://www.catholicapologetics.info/modernproblems/ecumenism/salvation.htm
Thanks for that post.
A very good explanation and exposition.