So, there are two types of ignorance, where it comes to the faith, formal and material (similar to the two types of heresy, for instance).
I could be a Catholic and am ignorant about a certain dogma or two the Church teaches, but I nevertheless intend to believe anything the Church teaches, just don't know about some of them. Someone in this category can have Catholic faith, though he may or may not sin depending upon whether there's some culpability, i.e. you were just too lazy to study your catechism, but it's also possible that you don't even know what you don't know, so never thought to look into it. God alone knows the degree of culpability.
Now, if I don't even know about the Church, and do not at least know about the minimum essential dogmas of the faith, i.e. the Holy Trinity and Incarnation, then I cannot have faith, and this is therefore formal ignorance. St. Pius X discussed this distinction somewhere, and so did the Holy Office in rejecting the notion that believe in a Rewarder God might suffice, declaring these truths to be necessary by necessity of means for supernatural faith and for salvation.
No amount of "sincerity" or "good will" or lack of culpability would supply for this second type of ignorance, for without this basic knowledge supernatural faith is impossible and someone cannot have the supernatural formal motive of faith. Same with Protestants, since they don't believe the Magisterium is the rule of faith, they lack the formal motive of faith, and their "sincerity" (such as it might be) cannot supply for a lack of this formal motive of faith.
Invincibility speaks only to culpability with regard to any ACTUAL SIN against faith, as St. Thomas teaches. If my ignorance is invincible, then I commit no sin, but it still does not mean that I can have supernatural faith without those bare minimums. As St. Thomas teaches also, if there is such a one who has placed no impediments of sin (whether culpable sin against faith or mortal sin against natural law, etc.) in the way of receiving faith, God will bring the faith to such a one, either by an internal illumination or sending an angel to teach the faith if necessary, or bilocating a Mary of Agreda or someone else to them, etc. Of course, the very same may be said regarding the Sacrament of Baptism. If God's already bringing an angel or bilocating some missionary to teach the faith, why couldn't that individual also perform the Sacrament of Baptism?
Now, many of the Cushingites claim that it's contrary to God's salvific will for all not to provide some implicit means of having supernatural faith and Baptism ... though of course, as I pointed out, that is not implicit BoD but implicit faith. That's nonsense. What about the unbaptized infant? Is it contrary to God's salvific will for all if such an infant never had a chance? No, in some cases, God will withhold this enlightenment out of mercy. God would know that this unbaptized infant, if he were to live, would end up in hell, so He grants the mercy of perfect natural happiness in limbo. That is why I believe that the number of abortions has been allowed to skyrocket. God knows that the vast majority of those growing up in this world will end up in hell. Similarly for some adult, God would know that if the person's "invincible ignorance" were lifted, he would either reject the faith, therefore meriting greater punishment for eternity, or perhaps, having receive it, might lose his soul anyway ... both situations in which his eternal fate would have been worse had God NOT broken through the invincibility of their ignorance.
Bishop Williamson, God rest his soul, was the first one to mention at one point God's withholding of various actual graces out of mercy, to prevent them from suffering on account of rejecting them.