Whether or not Luther promoted polygamy is relatively unimportant.
Why?
Because God actually ordered it!
To recapitulate and expand: Where did God ever censure the patriarchs or anyone in Israel for having more than one wife? Or in the New Testament for that matter? Neither David nor Solomon were censured, even though they ignored the counsel offered in Deuteronomy 17:17. And the only prohibition of polygamy in apostolic times applied to church leaders – one apparent reason being to prevent time conflicts between paying fair and proper attention to multiple wives, and devoting adequate time to ministering to the people under their care.
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As mentioned generally in Post #12:
Deuteronomy 25:5:
If brothers live together, and one of them dies and has no child, the wife of the dead shall not marry outside to a stranger. Her husband's brother shall go in to her and take her as a wife for himself, and perform the duty of a husband's brother to her.
Polygamy. God ordered it and codified it in the Law.
And Genesis 38:6-10:
6 And Judah took a wife for Er his firstborn, whose name was Tamar.
7 And Er, Judah's firstborn, was wicked in the sight of the LORD; and the LORD slew him.
8 And Judah said unto Onan, Go in unto thy brother's wife, and marry her, and raise up seed to thy brother.
9 And Onan knew that the seed should not be his; and it came to pass, when he went in unto his brother's wife, that he spilled it on the ground, lest that he should give seed to his brother.
10 And the thing which he did displeased the LORD: wherefore he slew him also.
God had already made His requirements clear in patriarchal times.
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From Post #5:
“Perhaps a back-fire of "Sola Scriptura" that Protestants hadn't counted on?...
...it's the back-fire thing.”
From Post #6:
“Sola Scriptura lands you in uncharted territory with every reader who wants to ignore tradition. Luther may have respected some of the saints and some of the church councils but he was in uncharted territory a lot of the time so he had to wing it for doctrines (and he did).”
Actually, the problem is that those who proclaim “Sola Scriptura” actually practice “Sola Scriptura As Modified By Selected Traditions”.
True and pure Sola Scriptura does not lead to “uncharted territory” nor to “backfire things”. It presents a coherent message which was appropriate at the time of the apostles. Learning what that message was, can only bring blessing, no matter what God-inspired additions or changes to doctrine and practice might have sprung into being thereafter.
Just what was that blessed apostolic “faith once and for all delivered to the saints”?