妙法蓮華經玄贊 5.302

5.302

贊曰：合念大乘因化。舍唯一門菩薩能出，若我頓授智慧、神通，為說大因以化二子，捨說三乘之方便者，二乘器小不能依行得度眾苦，門既狹小或當墮落。此意亦合密說怖事，無令為火之所燒害。

The Commentary says: This corresponds to considering conversion through the causes of the Great Vehicle. The house has only one gate through which the bodhisattvas can emerge. If I were suddenly to bestow wisdom and supernatural penetrations and preach the great causes to transform the two sons, abandoning the expedient means of preaching the three vehicles, the vessels of the two vehicles would be small, unable to practice accordingly and attain deliverance from the myriad sufferings. Since the gate is narrow and small, they might fall back. This also implicitly corresponds to speaking of terrifying matters, so that they are not burned and harmed by the fire.