By faith you obey. By faith you follow God's leading. By faith you get up and get going. By faith in God's gracious purpose you seek God's will. 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.