Which French ruler converted to Roman Catholicism in order to solidify royal control over all of France?

Philip II was the King of Spain and not a ruler of France. His reign was characterized by the expansion of Spanish influence and the Catholic Counter-Reformation, but he did not convert to Catholicism in the context of French politics or to solidify control over France.

Henry III was the King of France during the late 16th century and faced significant challenges from both Protestants and Catholics. Although he was a Catholic, his inability to stabilize France and unify the factions during his reign ultimately led to his downfall, rather than a decisive conversion that secured royal authority.

Catherine de Médicis was the mother of three French kings and served as a powerful regent. While she played a crucial role in the politics of religious conflict, she did not convert to Catholicism to consolidate power; rather, she sought to navigate between Protestant and Catholic factions, often attempting to broker peace.