The Latin title Rex Catholicissimus, anglicised as Most Catholic King or 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.
The best-known example of this title is the Catholic Monarchs (Los Reyes Católicos), which is used solely in reference to Isabella I of Castile and Ferdinand II of Aragon.
Habsburg Spain refers to Spain and the Hispanic Monarchy, also known as the Catholic Monarchy, in the period from 1516 to 1700 when it was ruled by kings from the House of Habsburg. In this period the Spanish Empire was at the zenith of its influence and power. During this period, Spain held many territories, including Pan-American continental holdings and the West Indies; European territories like the Low Countries, Italian territories, Portugal and parts of France; and the Philippines and other possessions in Southeast Asia. The period of Spanish history has also been referred to as the "Age of Expansion".
The Habsburg name was not always used by the family members, who often emphasized their more prestigious princely titles. The dynasty was long known as the "House of Austria". In some circumstances, the family members were identified by their birthplace. Thus, Charles V was known in his youth as Charles of Ghent. As king of Spain, he was known as Charles I of Spain and as emperor, Charles V (in French, Charles Quint). In Spain, the dynasty was known as the Casa de Austria, including illegitimate sons such as John of Austria and John Joseph of Austria. The arms displayed in their simplest form were those of Austria, which the Habsburgs had made their own, at times impaled with the arms of the Duchy of Burgundy (ancient), as seen on the arms of John of Austria. Calling this era "Habsburg", is, to some extent, a convenience for historians.
The Catholic Monarchs were Queen Isabella I of Castile (r. 1474–1504) and King Ferdinand II of Aragon (r. 1479–1516), whose marriage and joint rule marked the de facto unification of Spain. They were both from the House of Trastámara and were second cousins, as they were both descended from John I of Castile. To remove the obstacle that this consanguinity would otherwise have posed to their marriage under canon law, they were given a papal dispensation by Sixtus IV. They married on October 19, 1469, in the city of Valladolid; Isabella was 18 years old and Ferdinand a year younger. Most scholars generally accept that the unification of Spain can essentially be traced back to the marriage of Ferdinand and Isabella. Their reign was called by W.H. Prescott "the most glorious epoch in the annals of Spain."
Spain was formed as a dynastic union of two crowns rather than a unitary state, as Castile and Aragon remained separate kingdoms until the Nueva Planta decrees of 1707–1716. The court of Ferdinand and Isabella was constantly on the move in order to bolster local support for the crown from local feudal lords. The title of "Catholic King and Queen" was officially bestowed on Ferdinand and Isabella by Pope Alexander VI in 1494, in recognition of their defence of the Catholic faith within their realms.