This is from the Latin title Catholicissimus, which prior to Charles V was rendered "Most Catholic Monarch" and after Charles V was rendered Most Catholic Majesty. According to Wikipedia, it is awarded to monarchs who in the eyes of that Pope embody Catholic principles in their personal lives and state policies. The title then remains attached to the monarchs descended from the original holder in perpetuity, unless withdrawn by a Pope. The reigning monarchs of Europe who currently carry that title are the monarchs of Belgium, Luxembourg, and Spain. Historically, this title is often used to refer to the Catholic monarchs Isabella I of Castile and Ferdinand II of Aragon. And that is how you will see it used in secular history books.
Catholic monarchs are any monarchs who follow the Catholic faith. The term The Catholic Monarchs is a term that the pope used to give occassionally to monarchs who especially identified with the faith, the most notable examples being Ferdinand and Isabella of Spain.
Because Spain had the "most Catholic monarchs" who sought to preserve the faith in their country unspotted by protestant heretics.
Spain
the catholic monarchs of Spain
It started killing the people
Queen Isabella I of Castile and King Ferdinand II of Aragon, together known as the "Catholic Monarchs," ruled Spain in 1492.
the pope's wealth
To make England more Catholic
To make England more Catholic
The pope and several Roman Catholic monarchs.
It ruled Europe so it was strong everywhere.
Most of the monarchs in the ancient world had been overthrown by 750 b.C.