It is clear from the context that Jesus is talking about understanding and believing. This can be stated clearly and changed to active form. Alternate translation: "But whoever does not have understanding will lose even what understanding he thinks he has" or "But God will cause those who do not believe the truth not to understand even the little that they think they have understood" (See: [[rc://en/ta/man/translate/figs-ellipsis]] and [[rc://en/ta/man/translate/figs-activepassive]])