"One of Luther's most passionate sermons on the Holy Mother was preached on the Feast of the Immaculate Conception."
Its a great sermon, too. But I think you'll find that the the feast was not called the "Feast of the Immaculate Conception" back then.
It wasn't declared dogma yet was it? Thought that and Papal infallibility were declared such after Marian apparitions in the 1800s