By faith Abraham obeys . . . Obedience is faith in motion. Bonhoeffer means that you can't do God's will without trusting God and you can't really trust God without also seeking God's will. If you try to obey without also believing, you'll miss the sheer graciousness of God's claim on you. But if you try to believe without also obeying, you'll forget that God's grace really does make demands on you.
Grace is never cheap.
Bonhoeffer goes on to say that the reason our faith is weak is that our lives are not fully committed. If we feel as though our prayers are getting no higher than the tops of our heads, it may be that we've held back some part of our lives from God's rule. If we fear that we're losing the faith we once cherished, it may be that we've not made peace with some person we resent. If we've lost the joy of knowing God, it may be that we've turned attention back on ourselves. If we're having trouble believing, it's because we're not obeying.