You would be told: It indicates their lack of ability to believe even if they [appear to] believe outwardly. It is the difference between “mā Zayd sakhīyan (Zayd is not generous),” and “mā Zayd bi-sakhīyin:” the first refers to his capricious character, its meaning is Zayd is not actually being generous although he has the ability and is a generous person. While the second means, Zayd does not have the capacity to be magnanimous and is not a generous person, although he does good works.