In 2005 (or 2006 - I don't quite remember) I went to a retreat/conference for UU Buddhists. It was a wonderful gathering. John Daido Loori, who was the Abbot of the Zen Mountain Monastery, gave a talk in which he described this triad, or three legs to the stool of practice: great faith, great doubt, and great determination.
Being the Buddhist/Christian hybrid that I am, I think of these three a little differently than he did. And I would argue that these three are critical to a truly spiritual life, whatever one's particular tradition.
For faith, I'll just let Abraham Joshua Heschel speak for me:
Faith is sensitiveness to what transcends nature, knowledge, and will, awareness of the ultimate, alertness to the holy dimension of all reality... To have faith is not to infer the beyond from the wretched here, but to perceive the wonder that is here and to be stirred by the desire to integrate the self into the holy order of living. It is not a deduction but an intuition, not a form of knowledge, of being convinced without proof, but the attitude of mind toward ideas whose scope is wider than its own capacity to grasp.