Taken from his book, Christless Christianity:
“We are not called to live the gospel, but to believe the gospel and to follow the law in view of God’s mercies. Turning the gospel into law is a very easy thing for us to do; it comes naturally. That is why we can never take the Good News for granted.
Any form of doing the gospel is a confusion of categories. The law tells us what to do; the gospel tells us what God has done for us in Christ. When it comes to the question of how we relate to God, doing is the wrong answer …
The law tells us what God expects of us; the gospel tells us what God has done for us. So law and gospel are not inherently opposed, but when it comes to how we are saved, these two principles could not be more antithetical.”
Some relevant Bible passages for reference: Romans Chapters 1, 4, and 11 (but really the whole book). 1 Corinthians 15, Ephesians 2, 1 Peter 2, Galatians (whole book).