If, then, Scripture is not crystal clear, how is it that those of whom you boast are not only blind at this point, but rash and foolish enough to define and assert free choice on the basis of Scripture, as though it were quite positive and plain? I mean your numerous body of most learned men who have found approval in so many centuries down to our day, most of whom have godliness of life as well as a wonderful skill in divine studies to commend them, and some gave testimony with their blood to the doctrine of Christ that they had defended with their writings. If you say this sincerely, it is a settled point with you that free choice has assertors endowed with a wonderful skill in Holy Writ, and that such men even bore witness to it with their blood. If that is true, they must have regarded Scripture as crystal clear; otherwise, what meaning would there be in that wonderful skill they had in Holy Writ? Besides, what levity and temerity of mind it would argue to shed their blood for something uncertain and obscure! That is not the act of martyrs of Christ, but of demons!
The Bondage of the Will
Luther’s Works Vol. 33