The answer, lies within.
Here is Luther on Galatians 2:7:
It seems such a little thing to mix the law and the Gospel, faith and works; but this does more mischief than human reason can conceive, for it not only blemishes and obscures the knowledge of grace, but it also takes away Christ, with all his benefits, and utterly overshadows the Gospel.
The cause of this great evil is our flesh, which, being immersed in sins, sees no way of getting out except by works and therefore wants to live in the righteousness of the law and rely on its own actions. Therefore, it is utterly ignorant of the doctrine of faith and grace, without which it is impossible for the conscience to find rest and quietness.