The decision to go to the very end.
The most essential quality is perseverance, endurance, and a - how to put it? - a kind of inner good mood that helps you not to get discouraged, not to become sad, and to face all difficulties with a smile. There is an English word which expresses this very well - cheerfulness. If you can keep this within you, you resist much better, you fight much better, in the Light, these bad influences which try to prevent you from progressing.