I don't know what you mean by "invicible ignorance", Lionel Andrades, but I know even in the sspx we know sometimes (but this is rare) some people are saved whereas there are not Christian. It is said that they are people that would have convert if they had met a missionary during their life. In that case, we speak about "baptême de désir". I don't know what the expression in English (that would look like "baptism of desire"). These are really rare case. I think these people receive really specific grace that makes their conscience very similar to hours with probably a really deep desire of the Truth.
We have to remember thazt before the Christ, they were no baptism with water. However, many souls are with God now.