Ignorance and sin

[Banaticus]

From ask an apologist, forums.catholic-questions.org/showthread.php?t=297137

When a holy day falls on a weekend and is not dispensed, then Catholics must attend two Masses, one for the holy day and one for Sunday. But since you did what you thought was correct and did not intend to skip one of the obligatory Masses, you did not sin.
(emphasis added)

Is that latter part something new? Is that strictly in keeping with Catholic theology, that if someone “sins” in ignorance and does what they “thought was correct” at the time, that there is no actual sin? That concept has potentially large ramifications and it was my understanding that this concept was not a formal part of Catholic doctrine.

 

[cargau]

Full knowledge and cooperation of the will are necessary for an action to be considered a mortal sin. Of course, the act must be defined as grave matter, as well.

A serious matter could be mitigated to a venial sin if full knowledge and consent was not present.

Let’s say you are driving a new car. You applied the cruise control and set it to 60. You pass right through a construction zone but you never learned how to turn the cruise off. You were prevented from acting rightly due to your ignorance.

Or, It would be like dropping a big stone on my foot. You probably didn’t mean it but my toe still hurts. My guess is that you would feel bad and say “I’m sorry”.

In both of these cases, a sin still exists. But it might not be mortal due to the absence of knowledge and/or the will.