大乘起信論義記 1.137

1.137

用大者，謂隨染業幻自然大用、報化二身麁細之用，令諸眾生始成世善終成出世故也。下文顯之。何故唯言善，不云不善者？以不善法違真故，是所治故，非其用也。若爾，諸不善法應離於真。釋云：以違真故，不得離真，以違真故，非其用也。

"Greatness of function" refers to the spontaneous great functions that follow karmic illusions of defilement, the coarse and subtle functions of the two bodies of reward and transformation, causing all sentient beings initially to accomplish worldly wholesomeness and ultimately to accomplish the supramundane. The text below clarifies this. Why does it mention only "wholesome" and not "unwholesome"? Because unwholesome phenomena run contrary to the real and are that which is to be remedied; they are not its function. If that is so, should not all unwholesome phenomena be separate from the real? The explanation says: Because they run contrary to the real, they cannot be separate from the real; yet because they run contrary to the real, they are not its function.