As I understand it, a precondition in Catholic theology for a sacrament to be valid is that the one performing the sacrament must have the correct intent. In the case of baptism, it is said, even an atheist can perform it if he uses the correct matter and form and has the correct intent. My question is twofold:
1. How can the faithful know the intent of the clergy? What if a few hundred years ago some bishop didn't really intend to consecrate another bishop? Does that mean the second wasn't really a bishop? Does it mean the priests he ordained could not perform valid sacraments? That could be a lot of priests by today.
2. How can a person who doesn't believe in sacramental grace at all have the intent to bestow that grace on another person?
I JUST had this conversation with a Roman seminarian on Tuesday night. He insisted that a priest at the altar who does everything visibly but privately says to himself, "I intend NOT to consecrate this" although he says the consecration aloud and to all others it appears he is, then the Mass is invalid and the gifts remain only common bread and wine. I insisted that the priest's visible and external intent forces an internal intent - like trying to say, "I refuse to consummate this marriage" to yourself during the first intercourse with your wife. Obviously, the marriage is being consummated. Now, if you want to fail to consummate the marriage, you can refuse to do so or even substitute it with some perversion, but the act of it is done and in that, the actual intent is revealed, regardless of the pretended intent.
I don't think a non-believing priest, who is acting as Christ and on behalf of the Church, can cause a sacrament not to happen by pretending to himself he does not will it to happen as long as he acts publicly as if it is. A sacrament is, after all, an external manifestation of an inward grace. He may desire to thwart the Holy Spirit, but as long as he does not make that intent clear, his intent to do what the Church does is presumed.
I asked my Byzantine priest and he said that's a very Roman question, and in the East we wouldn't even think about why a priest would do such a thing. He also agreed with me that even if a priest were pretending to himself that he privately intended for a sacrament not to happen, it would still happen because visibly and publicly he is acting on the Church's behalf.
So that's the answer as far as I can tell. Although, if the priest spends the homily talking about how he doesn't believe in the Real Presence, and says, when I say the words nothing happens, then you can be sure that it's not a valid Mass, because he obviously and openly doesn't intend to consecrate it and is no longer doing what the Church intends. However, sacraments are presumed valid unless opposing evidence is presented - so a "private" intention not to consecrate can't really happen.