It is forgiveness that sets a man working for God. He does not work in order to be forgiven, but because he has been forgiven, and the consciousness of his sin being pardoned makes him long more for its entire removal than ever he did before.
An unforgiven man cannot work. He has not the will, nor the power, nor the liberty. He is in chains. Israel in Egypt could not serve Jehovah. "Let my people go that they may serve Me." was God's message to Pharoah (Exod. 8:1): first liberty, then service.
A forgiven man is the true worker, the true law-keeper. He can, he will, he must work for God. He has come into contact with the part of God's character which warms his cold heart. Forgiving love constrains him. He cannot but work for Him who has removed his sins from him as far as the east is from the west. Forgiveness has made him a free man, and given him a new and most loving Master. Forgiveness, received freely from the God and Father of our Lord Jesus Christ, acts as a spring, an impulse, a stimulus of divine potency. It is more irresistible than law, or terror, or threat.
- Horatius Bonar
HT: The Discipline of Grace, by Jerry Bridges
(By the way, this is the best book I've read all year!)