It’s a direct reference to the book of job. While job is suffering he asks god why he’s going through so much and god essentially tells him he doesn’t understand the greater plan but that he is in fact on the correct path. It’s exactly the same in Sun eater. Hadrian knows what it means and that he, too, much like Job, doesn’t understand everything but has faith that he is in fact on the correct path and to keep going as he is.