You can indicate that Isaiah wrote this. You can also translate it in an active form. Alternate translation: "as Isaiah the prophet wrote" (See: [[rc://en/ta/man/translate/figs-explicit]] and [[rc://en/ta/man/translate/figs-activepassive]])
Both of these phrases mean basically the same thing and are metaphors that refer to Jesus and his death on the cross. It was as if the people stumbled over a stone because they were disgusted when they considered Jesus' death on the cross. (See: [[rc://en/ta/man/translate/figs-doublet]] and [[rc://en/ta/man/translate/figs-metaphor]])
# believes in it
Because the stone stands for a person, you may need to translate "believes in him."