why isabel and fernando called catholics monarchs

hello we are raul and alvaro and we are talk about the name of catholics monarchs                                                                                                                                                                                                              The title of "Catholic King and Queen" was bestowed on Ferdinand and Isabella byPope Alexander VI in 1494    in recognition of their defence of the Catholic faith within their realms.
"Catholic monarchs" or "kings" can also be used in a generic sense (e.g., "the Pope had authority over Catholic monarchs..."); the particular or generic use can be distinguished from the context.                                                                                                                                                                                             the enlace is the following     

Catholic Monarchs - Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Catholic_Monarchs