Here, then, is something fundamentally necessary and salutary for a Christian, to know that God foreknows nothing contingently, but that he foresees and purposes and does all things by his immutable, eternal, and infallible will. Here is a thunderbolt by which free choice is completely prostrated and shattered, so that those who want free choice asserted must either deny or explain away this thunderbolt, or get rid of it by some other means.
The Bondage of the Will
Luther’s Works Vol. 34-36