The Latin title Rex Catholicissimus, rendered as Most Catholic King and Most Catholic Majesty was awarded by the Pope to the Sovereigns of Spain. It was first used by Pope Alexander VI in the papal bull Inter caetera in 1493.
One of the rights of a "Most Catholic" queen – either regnant or consort – is the privilège du blanc, meaning that she may wear white when meeting the Pope rather than the normal black used by other consorts and heads of state.
The best-known example of this title is the Catholic Monarchs (Los Reyes Católicos), used solely in reference to Isabella I of Castile and Ferdinand II of Aragon.
Similar titles
The monarchs of other countries have received similar titles from the pope:
References
Rex Catholicissimus Wikipedia(Text) CC BY-SA