To remove Protestant characteristics, influence, or religion from something; to reverse Protestantization.
From de- (reverse) + Protestantize (from Protestant + -ize, meaning to make Protestant). This would have appeared in historical discussions of religious change, particularly in European history.
Deprotestantize appears in historical texts about the Counter-Reformation—it describes Catholic efforts to restore or reclaim territories that had become Protestant, showing how -ize and de- allow us to express religious-historical changes.
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