Luther on the Summary of all Christianity

"Grace and peace, these two words embrace the whole of Christianity. Grace forgives sin and peace stills the conscience. The two devils that plaque us are sin and conscience, the power of the Law and the sting of sin. But Christ has conquered these two monsters and trodden them underfoot, both in this age and in the age to come. The world does not know this,therefore it cannot teach anything sure about how to overcome sin, conscience, and death. Only Christians have this kind of teaching and are equipped and armed with it, so that they can overcome sin, despair and eternal death. It is a teaching that is given only by God; it does not proceed from free will nor was it invented by human reason or wisdom. These two words, grace and peace, contain a summer of all of Christianity. Grace contains the forgiveness of sins, a joyful peace, and a quiet conscience. But peace is impossible unless sin has first been forgiven, for the Law accuses and terrifies the conscience on account of sin. And the sin that the conscience feels cannot be removed by pilgrimages, vigils, labors, efforts, vows, or any other works; in fact, sin is increased by works. The more we work and set to extricate ourselves from sin, the worse off we are. For there is no way to remove sin except by grace....Because the world does not understand this doctrine, it neither can nor will tolerate it. It brags about free will, about our powers, about our works, all these as means by which to ear and attain grace and peace, that is, the forgiveness of sins and a joyful conscience. But the conscience cannot be quiet and joyful unless it has peace through this grace, that is, through the forgiveness of sins promised in Christ.... Therefore your bones and mine will know no rest until we hear the Word of grace and cling to it firmly and faithfully." Treasury of Daily Prayer, July 12, 2015

Luther on The Limits of Human Reason

"Thus it is shown than human reason, the more it presumes to explore and fathom God's essence, works, and ways, goes farther and farther astray and finally comes to consider God as nothing and to believe nothing at all. Today there are more and more of such people who consider themselves great and wise. So it must go with all those who proceed apart from the bare Word and instead consult with reason about the articles of faith to see how they square with it. This is what has happened to those erring spirits of ours with regard to the Sacrament, Baptism, and other articles." LW 69:31-32