Judaism is often called a tradition of deed, not of creed. It is certainly true that Judaism emphasizes one’s actions. The Torah assumes that the heart will always be divided and no one can erase the negative thoughts or bad intentions that accompany us throughout our lives. We show our nobility not by being consistently pure in thought, but by choosing to act in accord with our higher ideals.
Action helps shape character. Habit is the means by which we bind ourselves. Even things unimaginable at one point in life can become habits later on: changes in exercise or diet are largely a function of sticking with something long enough that it grows to be who we are. Similarly we can become accustomed to prayer, to certain mitzvoth, to participating in community. Created by God, we then become self-creators, and our will joins hands with our habits.
When Israel spoke the words na’aseh v’nishma — we will do and we will hear, it was not simply a statement of faith but a statement about human nature. Our actions will shape our souls. By doing better we will grow to be better.