From a professor of this stuff:

Interestingly, it is not the Peace of Westphalia to which your commenter alludes. It is even cooler and nearly a century older. It is the Peace of Augsburg, 1555. That Peace ended the Schmalkaldic War and brought about the end of the first wave of religious wars in the Holy Roman Empire. It was at Augsburg that the famous phrase, “cuis regio, eius religio,” (whose region, their religion) was adopted. It was also, and this is the important thing to the author’s point, an attempt (largely successful) by Great Lords to lessen the power and influence of the Empire’s lesser nobility.