In Mark 12:27, itis written "He is not the God of the dead but the God of the living, yes you err
In Ex 3:15, God said to Moses, this shall you say to the Israel; The Lord, the God of your Fathers, of Abraham, of Isaac, and of Jacob, has sent me to you, this is my name forever and by this name I am to be remembered to all generations
Are Abraham, Isaac and Jacob still alive? is this the same faith with believers who died?
Jesus was talking with/to the Sadducees
The Sadducees were a religio-political party made up of wealthy, liberal, secular-minded people. They accepted only the five books of Moses, Genesis to Deuteronomy, as inspired, and did not believe in the resurrection of the dead.
Now they came to Jesus with a trick question that they felt sure would embarrass Him. No doubt it was a stock scenario that they had used in arguing with the Pharisees and others against the doctrine of the resurrection (yet, if you really look at their argument, it was kind of ridiculous: a woman who marries seven brothers, one right after the death of the other?). If Jesus disagreed with them, the Sadducees thought they could hold Him up to ridicule; but if He agreed, that would further offend the Pharisees.
In Jesus' own teachings to the people, He made the same point of this argument: Those who believe in Him, such as did Abraham, Isaac, and Jacob, have already crossed over from death to life and will surely come forth from the grave at His call (the 1st resurrection)
(John 5:24, 25). Even though our body turns to dust, we are bound up in the life of God, and we will live again. No wonder, then, for us death is only a sleep, a quiet rest for those whom, though in the grave, God considers as "living."
Also see ...
John 11:26; 1 John 5:11, 12.