“But if it is by grace, it is no longer on the basis on works; otherwise grace would no longer be grace.” (ESV)
Our insistence on salvation by grace alone must be absolute. If we allow even a smidgen of works in the door the whole thing comes undone. Grace flies out the window and doesn’t return until we drive out works completely. When it comes to salvation, grace and works are mutually exclusive. You can have one or the other, but not both. Period.
This is hard for us to get our brains around because we naturally want to be able to have something to do with our salvation. That’s our sinful nature at work. Wanting to take ownership of our relationship with God was part of how sin entered the world in the first place.
The fact of the matter is: There is nothing we can do to gain a right status with God once we have lost it. Nothing. Not a single thing. Nor will there ever be. Salvation is entirely by grace or it is entirely by works. This is simply a matter of definitions. Grace is unmerited favor. If there’s something we can do to merit it…then it’s not grace. And if salvation is by works, then it is something God owes us. God doesn’t owe us anything. We owe Him everything. We work as Christians, yes, but we work because of grace, not to get it.