Possible meanings are 1) to explain the scriptures in a way that people can understand is spoken of as if Paul were opening something so people can see what is inside of it) or 2) Paul was literally opening a book or scroll and reading from it. (See: [[rc://en/ta/man/translate/figs-metaphor]])
From among all those who have died. The expression "the dead" describes all dead people together in the underworld. To come back from among them speaks of becoming alive again.