«العدالة: وهي الاستقامة في الدين»
ʿAdalah means uprightness in religion.
The book treats major sin or persistence in minor sin as matters affecting uprightness. The underlying point is that trustworthiness in transmitting religious reports involves character and truthfulness, not just retention.
Even a person who memorizes a report accurately but is known to deliberately falsify statements would not become reliable merely through strong memory. Uprightness and precision are independent considerations.