For a while, it was doubtful whether France would have a Catholic or a Protestant king.
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Actually by law the king had to be catholic...
That’s why Henri IV had to become a Catholic..
Well, the law was changeable. And in fact the agreement that the various factions arrived at after the 30 Years’ War was “Cuius regio, eius religio.” I.e., whatever the king’s religion, that shall be the religion of the people.
But Henri IV did indeed choose the expedient course. It was what his key supporters wanted. As he said, “Paris vaut bien une messe.”