Luther Responding to Erasmus:
THE “Form” of Christianity set forth by you (Erasmus), among other things, has this — “That we should strive with all our powers, have recourse to the remedy of repentance, and in all ways try to gain the mercy of God; without which, neither human will, nor endeavour, is effectual.” Also, “that no one should despair of pardon from a God by nature most merciful.” —
These statements of yours are without Christ, without the Spirit, and more cold than ice: so that, the beauty of your eloquence is really deformed by them. Perhaps a fear of the Popes and those tyrants, extorted them from you their miserable vassal, lest you should appear to them a perfect atheist. But what they assert is this — That there is ability in us; that there is a striving with all our powers; that there is mercy in God; that there are ways of gaining that mercy;
Here you plainly assert, that the will does something in those things which pertain unto eternal salvation, when you speak of it as striving: and again, you assert that it is passive, when you say, that without the mercy of God it is ineffective. Though, at the same time, you do not define how far that doing, and being passive, is to be understood: thus, designedly keeping us in ignorance how far the mercy of God extends, and how far our own will extends; what our own will is to do, in that which you enjoin, and what the mercy of God is to do.
-- Martin Luther, De Servo Arbitrio