妙法蓮華經玄贊 2.6

2.6

不退有四：一信不退，十信第六名不退心，自後不退生邪見故。二位不退，十住第七名不退位，自後不退入二乘故。三證不退，初地以上即名不退，所證得法不退失故。四行不退，八地已上名不退地，為、無為法皆能修故。今此菩薩皆八地已上故，言於無上正等覺不退轉，定當證故，故不退者非即不轉。

There are four kinds of non-retrogression: first, non-retrogression of faith: the sixth of the ten faiths is called the non-retrogressing mind, because thereafter one does not retrogress to generate wrong views. Second, non-retrogression of stage: the seventh of the ten abidings is called the non-retrogressing stage, because thereafter one does not retrogress into the two vehicles. Third, non-retrogression of realization: from the first stage onward is called non-retrogression, because the dharmas realized are not lost. Fourth, non-retrogression of practice: from the eighth stage onward is called the non-retrogressing stage, because one is capable of cultivating both conditioned and unconditioned dharmas. Now, these bodhisattvas are all at or above the eighth stage, so the text says that they are non-retrogressing with respect to unsurpassed right and equal awakening because they are certain to realize it; thus "not retrogressing" (butui) does not merely mean "not turning back" (buzhuan).