Here are some extracts from Luther in his exchange with Erasmus, taken of course from The Bondage of the Will:
For not to delight in assertions, is not the character of the Christian mind: nay, he must delight in assertions, or he is not a Christian. But (that we may not be mistaken in terms) by assertions, I mean a constant adhering, affirming, confessing, defending, and invincibly persevering.
And moreover, I speak concerning the asserting of those things, which are delivered to us from above in the Holy Scriptures.
But why should I dwell upon this; nothing is more known and more general among Christians than assertions. Take away assertions and you take away Christianity.
Whereas, the Christian will rather say this--I am so averse to the sentiments of the Sceptics, that wherever I am not hindered by the infirmity of the flesh, I will not only steadily adhere to the Sacred Writings everywhere, and in all parts of them, and assert them, but I wish also to be as certain as possible in things that are not necessary, and that lie without the Scripture: for what is more miserable than uncertainty.
The Holy Spirit is not a Sceptic, nor are what he has written on our own hearts doubts and opinions, but assertions more certain, and more firm, than life itself and all human experience.