大乘起信論別記 1.90

1.90

「能知名義為說真覺」者，由妄想心知名義故，方有言說，說於真覺，是名能說真覺之言，要依不覺所起妄想。

As for "being able to know names and meanings, and expounding true enlightenment for [sentient beings]," because the deluded mind knows names and meanings, only then is there conventional speech expounding true enlightenment; this is named speech capable of expounding true enlightenment, which must rely on the deluded thoughts produced by non-enlightenment.