"risen from among the dead ... to rise from among the dead." This speaks of becoming alive again. The phrase "the dead" refers to "dead people" and is a metonym for death. Alternate translation: "risen from death ... rising from death" (See: [[rc://en/ta/man/translate/figs-metonymy]])
Here "kept the matter to themselves" is an idiom that means they did not tell anyone about what they had seen. Alternate translation: "So they did not tell anyone about what they had seen" (See: [[rc://en/ta/man/translate/figs-idiom]])