en_tn/mrk/12/27.md

not the God of the dead, but of the living

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/jit/figs-nominaladj and rc://en/ta/man/jit/figs-ellipsis)

the living

This includes people who are alive physically and spiritually.

You are quite mistaken

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/jit/figs-explicit)

quite mistaken

"completely mistaken" or "very wrong"