妙法蓮華經玄贊 2.296

2.296

贊曰：第三善堅實如來法輪因也。論云：「佛滅度後無量時說，故教化之令其堅固，即入八地。」堅固乃是不退義故，不爾如何今已成佛？或入初地得不壞信名堅固故。

The commentary states: The third is the cause of excellently solidifying the Tathāgata's wheel of the Dharma. The treatise says: "Because he preached for an immeasurable time after the Buddha passed into extinction, he taught and converted them to make them firm, so that they entered the eighth stage." Firmness means non-retrogression; otherwise, how could they have already attained buddhahood now? Alternatively, it is called firm because entering the first stage one attains indestructible faith.