Romans 4:4-5

Now to him who works the wage is not accounted according to grace, but according to debt; yet to him who is not working, yet believes upon Him who justifies the ungodly, his faith is accounted to righteousness”

When someone works, he earns his reward; his reward is deserved. Grace is the extension of some benefit which is not deserved. There is no grace in paying someone what they have earned; the wages are owed; they must be paid.

When someone does not work, he earns nothing. Any benefit he receives is undeserved, and imparted through the grace of the one who gives. A man who does not work to earn God’s benefits, but trusts God, has acknowledged God as God when he believes what God has said. He has nothing to offer God to compel God to impart to him divine benefits, and may therefore expect only what he is owed: nothing. But God has declared that He will justify the ungodly on the basis of their faith. Therefore the man who believes God receives His grace; his debt is paid and God declares him clean. The sins of the past are paid, and that man becomes righteous in the eyes of the Sovereign LORD.