-— So Luther believed only the Bible could be used in knowing God’s will -—
Which Bible?
There was no infallible indisputable decree on the canon by Rome till the year Luther died, and whether Luther believed only the Scriptures could be used in knowing God’s will is the issue, while the means by which writings are established as Scripture is another.
Is a assuredly infallible magisterium necessary for this, and does being the steward of Divine revelation, and inheritor of promises of God's presence and preservation, and having historical descent, require or render such to be that infallible interpreter to whom all must submit?