John Gill Commentary Mark 12:27

"He is not the God of the dead, but of the living: ye do greatly err." — Mark 12:27 (ASV)

He is not the God of the dead
This is our Lord's reasoning upon the passage; showing from hence, that since God is the God of these persons, they must be now alive in their souls, for God is not the God of the dead; and that their bodies must rise again, or he will not be the God of their whole persons;

but the God, of the living :
the word "God", in this clause, is omitted in the Vulgate Latin, Syriac, and Arabic versions, but retained in the Persic and Ethiopic versions; (See Gill on Matthew 22:32);

you therefore do greatly err ;
signifying, that it was not a slight mistake, an error of small importance, but a very great one; inasmuch as it was contrary to the Scriptures, and derogated from the power of God, and destroyed that covenant interest, which God has in his people, and particularly in the principal men of their nation, who were the fathers and founders of them.