"These two words, grace and peace, do contain in them the whole sum of Christianity. Grace containeth the remission of sins; peace a quiet and joyful conscience. For peace of conscience can never be had, unless sin first be forgiven. But sin is not forgiven for the fulfilling of the law; for no man is able to satisfy the law. The law doth rather show sin, accuse and terrify the conscience, declare the wrath of God, and drive to desperation. Much less is sin taken away by the works and inventions of men, but sin is rather increased by works. So there is no means to take away sin, but grace alone. Therefore Paul in all the greetings of his epistles, sets grace and peace against sin and an evil conscience."
Martin Luther - "Commentary on Galatians" - Kregel (1979) - p6.