Justify = declare or make righteous in the sight of God
I think the way Paul and James’ teaching fit together is that Paul’s talking about how we’re made righteous, while James seems to be talking about how we’re declared righteous. Jesus said a “tree is known by its fruit”. (Matt 12:33) “No good tree bears bad fruit, nor again does a bad tree bear good fruit.” (Luke 6:43) If you’re a righteous tree, you’ll bear righteous fruit, but if you’re not a good tree, trying to stick good fruit onto your branches won’t make you into a good tree. You need to be good from the roots up.
If we are righteous, our works will declare us to be righteous – they’ll “justify” us – and the book of James especially warns us that if our works don’t declare us to be righteous then we really need to examine whether we are righteous, but that doesn’t answer the question: “how do we become righteous?”
Paul spends v10-12 on what doesn’t make us righteous. And he doesn’t just say “those who rely on works are mistaken about this particular aspect of Christian teaching” or “if you rely on works you’re going to struggle with assurance of your salvation”. He says “all who rely on works of the law are under a curse”. This is serious stuff. It’s not just a case of some Christians not quite getting the right balance between grace and law and leaning towards legalism is causing them a few problems.
As far as the basis of our righteousness is concerned, there’s no such thing as the right balance between grace and law. If you’re basing your righteousness on works of the law, Paul says you’re under a curse.
And he quotes Deuteronomy 27:26 “Cursed be everyone who does not abide by all things written in the Book of the Law, and do them.” – and James says in Chapter 2v10 “whoever keeps the whole law but fails in one point has become accountable for all of it” also Galatians 5:3 says “I testify again to every man who accepts circumcision that he is obligated to keep the whole law” – It’s no good to think to ourselves “well I may have broken some parts of the law, but I’m doing ok in other areas. I’m certainly not as bad as that person over there”. If any part of our hope of being counted righteous by God rests on something we’ve done, we might as well say to God “I want this work that I’ve done to be counted towards my righteousness… I want you to take my law-keeping into account”, and if that’s the case, God will hold us accountable for the whole law, and then we’ll be stuffed. It has to be all Christ and nothing but Christ.