天台四教儀 1.18

1.18

第三祕密教者，如前四時中，如來三輪不思議故，或為此人說頓、或為彼人說漸，彼此互不相知，能令得益，故言祕密教。

Third, the secret teaching: as in the preceding four periods, because the Tathāgata's three wheels are inconceivable, he would preach the sudden for this person or preach the gradual for that person, while they remained mutually unaware of each other, yet were enabled to obtain benefit; therefore it is called the secret teaching.