Here "the dead" refers to people who are dead, and "the living" refers to people who are alive. Also, the words "the God" can be stated clearly in the second phrase. Alternate translation: "not the God of dead people, but the God of living people" (See: [[rc://en/ta/man/translate/figs-nominaladj]] and [[rc://en/ta/man/translate/figs-ellipsis]])
It may be helpful to state what they are mistaken about. Alternate translation: "When you say that dead people do not rise again, you are quite mistaken" (See: [[rc://en/ta/man/translate/figs-explicit]])