This implies that he was permitting them to tell people about what they had seen only after he rose from being dead. (See: [[rc://en/ta/man/translate/figs-explicit]])
"risen from the dead ... to rise from the dead" or "risen from among the dead ones ... to rise from among the dead ones." This means to become alive again. The phrase "the dead ones" refers to "dead people" and is a metaphor for being dead. AT: "risen from being dead ... to rise from being dead" (See: [[rc://en/ta/man/translate/figs-metaphor]])
Here "kept the matter to themselves" is an idiom that means they did not tell anyone about what they had seen. AT: "So they did not tell anyone about what they had seen" (See: [[rc://en/ta/man/translate/figs-idiom]])