It is going to be tough to find an unbiased history on anything, especially anything regarding the History of Christendom. Even secular authors have their biases and opinions on it.
I recommend from the Catholic perspective:
Belloc’s How the Reformation Happened and Warren Carroll’s History of Christendom series.
From the Protestant perspective
Alister Mcgrath’s Christianity’s Dangerous Idea and (though outdated a bit) Ranke’s The History of the Popes and his History of the Reformation in Germany.
Although all four historians have their biases and opinions, this will give you a good mix of views from both sides and their works include a great deal of primary sources that you can follow up on and find your own views on.
I forgot to add NMHS, that before you read the History, take time to read the Historian. This will give you a good idea of the biases and slants that each historian will be coming in with.
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