From his commentary on the book of Romans, specifically chapter 14, written in 1515.
Nor does it belong to the new Law to build such or such churches, to adorn them so or so, or to sing in this way or another. Nor are organs, altar decorations, chalices, and pictures necessary, and whatever else we now find in the houses of worship. Nor is it necessary for priest and monks to be shaved or appear in special garments as in the Old Testament. All this is "shadow" and "token" of the new reality and puerility.
Wow. Luther was forward thinking.
I'm trying to sift Romans 14. Paul seems to agree with the strong but suggests we do church as the weak desire. But this isn't his point. I can come up with obvious contradictions. I think it is more about Paul's image of us leading others toward Christ, in a humble, non-pushy, self-discovery full of encouragement.
Maybe I should go back to that Merton post.