NOTES for Mar 12:27
The Scriptural text that God is not God of the dead but of the living is cited by Jesus as proof of the inevitability of the general resurrection, and not only His own. If we consider this verse in its immediate meaning, we will find nothing of the kind in it: it means only that God is the God of life, not the God of death, and that He leads a person by the path of life, delivering him from death.
Usually this was understood in the context of a person's earthly life: it was assumed that God could, for example, deliver a righteous person from a deadly danger threatening him, including in order to demonstrate to skeptics that He is truly ready to intervene in events and support His witness.
At the same time, however, it was taken for granted that all people are mortal, and even a righteous person who has been granted a happy and long life will still one day die. Such a death was and is called "natural," and although in fact few receive it, in the Old Testament books it was considered the standard, so to speak, the "ideal death": a person lived the life of a righteous man and departed peacefully, leaving an inheritance to his children, whom he had, of course, already had time to raise as righteous as he himself was. Later the righteous awaited resurrection, but this was an event separate from their earthly life.
Jesus, however, says that death is not natural by definition: for God is the source of life and life itself. He did not create death; it became the consequence of the fall, which distorted creation. Against such a background, resurrection is not something accidental, not an ordinary miracle of God that nevertheless might not have happened. It is part of that plan of universal salvation and transformation of creation whose culmination was the coming of the Messiah.
In God's Kingdom there can be no death precisely because it is God's Kingdom, and life, every earthly life, has its foundation in the eternity of this Kingdom. Life can have no natural, physical sources, although it can and does have a natural, physical projection. Life itself is a spiritual process, and what in our world limited by space and time is perceived as a short stretch of time abides in the eternity of the Kingdom as the fullness, revealed to one degree or another depending on the spiritual quality of the life lived, of a concrete person who will remain there forever.
Or will not remain, if there was nothing in that person's life from the Kingdom and from that breath of God with which it is permeated. Resurrection is not simply a return to life, but a return to life in all its fullness, in the fullness of the Kingdom. That is why it can turn out for a person to be either joy or suffering. And what will await him in eternity depends only on the choice the person himself must make here, in this life.