I think the voice is the devil.
Because we, as laymen, are not called to enter into profound matters of theology.
To enter into such matters properly, with minimal chance of error -- being led astray by pride, desire to sleep in on Sunday, etc. -- , we would have to STUDY theology, not just read a book or two. That would take 3 years minimum, which is what priests spend. Even that is considered a START. Priests whose office is to teach, etc. will study a lot more than 3 years.
If Jack simply followed his priest, trusting that the priest knew best, God would not hold him accountable. If Jack were objectively right, the PRIEST might be held accountable...
That is where some sedevacantists get things wrong.