If we do not trust in the power of our meditation to heal, its strength and energy will be weak. Belief gives the meditation a firm foundation; it engages the mind in a way that is effective and total. This is not blind faith, but a faith and trust based on knowledge that the healing power of mind can be fully called forth with the help of images, words, and feelings. We need to believe that we actually can improve our lives in this way. Even if meditation moves us one step forward, we can fall right back if we are always harboring doubts in our minds.
From Boundless Healing