That's interesting what you posted. However, wasn't Henry IV of France forced to become Catholic before the Pope ruled that he was the legitamite king?
Yes, Henri IV had to convert. But that was French law, not the Pope's. There is a difference between monarchies in Catholic countries, where the succession is properly restricted to Catholics (as it still is in Spain), and monarchies in predominantly non-Catholic countries like Russia, where non-Catholic sovereigns can legitimately rule and are entitled to the allegiance of their Catholic subjects as long as they do not attempt to force them to renounce their faith or otherwise sin.