大般涅槃經集解 21.33

21.33

案僧亮曰：字義者，雖復讀誦文字，而行違說旨者，字則不為是人出也，名為無字，未知常住佛性故也。

Sengliang comments: Regarding the meaning of characters: even if one recites the written characters, if one's practice goes against the intended teaching, the characters do not manifest for this person; this is called 'having no characters', because they do not yet know the eternally abiding buddhadhātu.