Man's life is not in himself. It is the power of God by which he lives, and which returns to God when man dies. If God were to gather it all to Himself, man would perish from the universe. In whose hand is the soul of every living thing, and the breath of all mankind. Man has no existence of any kind in death.
Man is mortal because of sin.
It is God's law that sinners must die. Adam, our first father, sinned and was sentenced to death before he had any children. Death began with him, and came to us through him. We receive the nature that he had after he was condemned to die. We thus inherit his sentence of death.