believers need the law

the Formula of Concord, Solid Declaration VI:

For this reason, too, believers require the teaching of the law: so that they do not fall back on their own holiness and piety and under the appearance of God’s Spirit establish their own service to God on the basis of their own choice, without God’s Word or command. As it is written in Deuteronomy 12[:8,28,32], “You shall not actall of us according to our own desires,” but “listen to the commands and laws which I command you,” and “you shall not add to them nor take anything form them.” Furthermore believers also require the teaching of the law regarding their good works, for otherwise people can easily imagine that their works and life are completely pure and perfect [FC-SD VI, 20,21].