Why do Jews believe in works instead of grace?
They don’t. This is a misrepresentation on the part of Christianity. Gentiles who had no intention of becoming Jews, yet wanted to devise a faith in the Jewish god, needed a rationale for doing so. Due to a Gnostic influence which said that the physical world was flawed and evil, the Pauline Christians rejected physical requirements like circumcision, and law keeping, and instead emphasized spirituality. In maintaining a Gnostic worldview they also demonized Jews, and some claimed that Yahweh was evil. Jesus was seen as the new spiritual god who had replaced Yahweh and physical Jewish religion. In other words, all the things Yahweh commanded the Hebrews to do, according to Gnostic thought, were physical and thus evil. God (in their minds, Jesus) could only be appeased by some spiritual act to remove them from the physical realm where sin (evil) ruled. Thus the Christian doctrine of works versus grace came about from the Gnostic doctrine of spiritual versus physical.
