When Adam ate of the Tree of Knowledge of Good and Evil, he became mortal just as God had warned. Once Adam and Eve had become mortal, their children were also mortal. This human mortality has passed from generation to generation ever since. The law simply describes how God's moral universe operates and serves as a guide to keep us out of trouble. It is not possible for good behavior - obeying the law - to confer immortality. That can only be done by God as a gift of grace.