How did Catholic start in Spain?

Answer 1

As Spain explored the new world, they brought along priests, monks, and friars with them, every exploring ship had its religious on board. Remember that the King and Queen of Spain were extremely pious religious and sought the advancement of Christianity above all else.

Answer 2

Catholicism came very early to Spain, along with the Romans, who conquered it. Below is an extract from the Catholic Encyclopedia, you may read the entire article at the link below.

Under the Roman domination Spain received Christianity. There is a venerable tradition that the Apostles Paul and James came to the country, as well as the Seven Apostolic Men (Torquatus, Ctesiphon, Secundus, Indalecius, Caecilius, Hesychius, and Euphrasius) to whom the foundation of various churches is attributed. Connected with the coming of St. James is the very ancient tradition of Our Lady of the Pillar (la Virgen del Pilar) of Saragossa. Prudentius says that there were martyrs in Spain in every one of the persecutions. Of uncertain date are the martyrdoms of Sts. Facundus and Primitius in Galicia; of St. Firminus and Sts Marcellus and Nonia, with their twelve children, in León; of Sts Acisclus and Victoria at Cordova. Sts Hemererius and Celedonius suffered in the Decian persecution, as did Sts. Justa and Rufina, St. Laurence, St. Fructuosus, St. Augurius, and St. Eulogius. The most famous of Spanish martyrs, however, are those who suffered in the persecution of Diocletian, when Dacian was prefect; among them were Sts. Cucufatis, Eulalia, and Severus, Bishop of Barcelona, Sts. Félix, Poncius, and Victor, Narcissus, Bishop of Gerona, Engratia, Valerius, Bishop of Saragossa, and his deacon, Vincentius, Justus and Pastor of Alcalá, Leocadia of Toledo, Eulalia of Mérida, Cyricus and Paula of Malaga, Vincentius, Sabina, and Cristeta of Talavera. During this period, too, many councils were held in Spain, the most important being those of Elvira (or Illiberis) and of Saragossa, and the First Council of Toledo. At that of Elvira (300) the Acts, which are still extant, were signed by nineteen bishops, and, among other things, the celibacy of the clergy was insisted upon. At the Council of Saragossa (380) Priscillianism was condemned. The Priscillianists abjured their heresy at the Council of Toledo (400), where, also, the symbol was pronounced with the Filioque. Among illustrious Spaniards of the period may be mentioned Pope St. Damasus, the great Hosius, St. Pacianus, Bishop of Barcelona, and his son, Flavius Dexter, Juvencus and Prudentius.