Now to him that worketh is the reward not reckoned of grace, but of debt.
Note 7 at Ro 4:4: Paul was saying that if an individual could be saved by works, then God would be providing salvation as a payment to that person, and of course, that doesn't make sense. God is not under obligation or debt to save anyone. Trust in our own works voids grace, and likewise, trust in God's grace makes faith in our own efforts useless. This was repeated by Paul again in Ro 11:6--"And if by grace, then is it no more of works: otherwise grace is no more grace. But if it be of works, then is it no more grace: otherwise work is no more work."