Paul's opponents have been arguing against him, and for their own position, on the basis of their understanding of the Old Testament (particularly the first 5 books called "The Law"). Paul flips the script as he challenges those abandoning grace for self-righteousness. He acknowledges the law and contends it actually works against those who try to use it to claim any self-righteousness. Our only hope is grace alone, through faith alone, in Christ alone. And praise God, that hope is available to the barren, the failures, and the misfits.