....because they mingle the law with the Gospel, they must needs be perverters of the Gospel.  For either Christ must remain, and the law perish, or the law must remain and Christ perish; for Christ and the law can by no means agree and reign together in the conscience.  Where the righteousness of the law ruleth, there cannot the righteousness of grace rule; and again, where the righteousness of grace reigneth, there cannot the righteousness of the law reign; for one of them must needs give place unto the other.  And if thou canst not believe that God will forgive thy sins for Christ's sake, whom he sent into the world to be our high-priest; how then, I pray thee, wilt thou believe that he will forgive the same for the works of the law, which thou couldest never perform; or for thine own works, which (as thou must be constrained to confess) be such, as it is impossible for them to countervail the judgment of God?
     Wherefore, the doctrine of grace can by no means stand with the doctrine of the law.  The one must simply be refused and abolished, and the other confirmed and established.  For as Paul saith here, to mingle the one with the other, is to overthrow the Gospel of Christ.  And yet, if it come to debating, the greater part overcometh the better; for Christ, with his side, is weak, and the Gospel but a foolish preaching; contrariwise, the kingdom of the world, and the devil, the prince thereof, are strong.  Besides that, the wisdom and righteousness of the flesh carry a goodly show; and by this means, the righteousness of grace and faith is lost, and the other righteousness of the law and works advanced and maintained.....
     It seemeth to be a light matter to mingle the law and the Gospel, faith and works, together:  but it doth more mischief than a man's reason can conceive; for it doth not only blemish and darken the knowledge of grace, but also it taketh away Christ, with all his benefits, and it utterly overthroweth the Gospel, as Paul saith in this place.  The cause of this great evil is our flesh, which, being plunged in sins, seeth no way how to get out but by works, and therefore it would live in the righteousness of the law, and rest in the trust and confidence of her own works.  Wherefore, it is utterly ignorant of the doctrine of faith and grace, without the which, not withstanding, it is impossible for the conscience to find rest and quietness....
.....So we at this day do not reject fasting, and other good exercises, as damnable things; but we teach, that by these exercises we do not obtain remission of sins.
                                                                                                                                             Martin Luther