妙法蓮華經玄贊 2.7

2.7

又不退有二：一已得不退，初地即得；二未得不退，八地方得。無上正覺是未得法故，八地以上能不退轉，情祈正覺心進不動，法駛流中任運轉故，名不退轉，此不退者即是不轉。

Furthermore, there are two kinds of non-retrogression: first, non-retrogression regarding what has already been attained, which is obtained at the first stage; second, non-retrogression regarding what has not yet been attained, which is obtained only at the eighth stage. Since unsurpassed right awakening is a dharma not yet attained, from the eighth stage onward they can be non-retrogressing: with minds yearning for right awakening, they advance without wavering, spontaneously advancing in the swift current of dharmas, which is called non-retrogressing; here "not retrogressing" is synonymous with "not turning back."