Rom 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.
Is the whole of our salvation accomplished by the finished work of our Holy Substitute? Or, is there something else that man can do to make himself more fit, qualified, holy, or otherwise more acceptable in the sight of God? Can man establish his righteousness more securely by his supposed performance of the law? Or, is the righteousness of Christ our all and only means of acceptance before a Holy God? The message of free and sovereign grace is an offense to men's pride because they refuse to admit that they are totally incapable of doing anything good (Rom. 3:10-12). Men, refusing to find their satisfaction in the righteousness of Christ, go about to establish their own righteousness (Rom. 10:3). Though they often succeed in impressing other men, most especially themselves, God is worse than not impressed. He is infinitely offended by their attempt to rob Christ of His glory.
God says works and grace are mutually exclusive. Grace by definition means the total absence of works. Once you introduce works into grace you no longer have grace but only works. A little bit of leaven leavens the whole lump. You can not mix works and grace any more than you can mix oil and water. Regardless of how hard you shake the bottle the heavier water gravitates to the bottom and the lighter oil to the top. So it is with works and grace. The living water of God's grace will be completely covered by the oil of man's works, so that if one attempts to find refreshment from that glass they will ingest nothing but sickening oil. Not only will it not satisfy, it will cause them to loose even what they have.