"Ojalá" is a relatively common word in modern-day Spanish. I've always been taught that it originated during the Moorish rule of Iberia during the medieval era, deriving from the Arabic expression "May Allah will it." This seems a bit strange, given the animosity of Spanish Christians at the time towards their Muslim rulers and later the persecution of Muslims under the first monarchs of a reunited Spain, Ferdinand II and Isabella I. Why did the word "ojalá" persist in the vocabulary of Spanish Christians, despite its Islamic roots? Was their any pushback to its use?
While it is hard to specifically answer why "ojalá" persisted in the vocabulary, there definitely was pushback to its use.
There was pushback in general to the use of Arabic-derived words as Castilian (what is generally called Spanish in English) was being standardized after the Reconquista. Castilian was considered "a symbol of national unity" and "Arab influences were seen as a pollutant of Castilian purity" (González Sopeña 111).
For example, López de Villalobos (1544:434) stated
"the clergy... do not say albaceha, nor almutacen, nor atafoico, nor other Moorish words... which dirty and darken the polish and clarity of the Castilian language."
Martín de Viciana (1574:16) states his disapproval of Arabic loanwords in Castilian because "it comes from the enemies of our Holy Christian Religion."
However, basically, the Reconquista was a 770-year process lasting from . A standardized Castilian Spanish was not even around when Iberian Christians began to successfully drive out the Moors. The version spoken around the city of Toledo was chosen by Alfonso X to be the basis for standardization, in the 13th century. The first standardized grammar was not completed until 1492. With the expulsion of the Moors, there were some successes in regards to "de-Arabicizing" the official vocabulary. The Biblioteca Virtual Miguel de Cervantes notes that after 1609 (when the Moors were expelled from Iberia), the word for "tailor," which had been the Moorish "alfayate" or "alfageme," began being called "sastre"; "albeitar" fell out of favor to "veterinario" (veterinarian), etc.
However, many Arabicized terms such as ojalá were already engrained in local linguistic patterns and simply stayed, despite elite attempts to remove them from Christian Spain's vocabulary. Lapesa (1981:156) summarizes that:
"Other (words) were only maintained in regional speech, but the vast majority of (Arabic-derived words) which survive today, many fundamental (parts of speech), characterize the Hispano-Portuguese lexicon against other Romance languages."
As a side note, modern-day Spanish speakers are often unaware of the roots of the word ojalá (at least here in Chile).
Sources:
González Sopeña, I. (2017). Arabismos y fiscalidad en el Reino de Granada, 35, Dicenda. Cuadernos de Filología Hispánica, 109-130.
"La invasión árabe. Los árabes y el elemento árabe en español." Biblioteca Virtual Miguel de Cervantes.
Lapesa, R. (1995): Historia de la lengua española, Madrid, Gredos (8.ª reimp. de la 9.ª de corr. y aum. 1981; 1.ª ed. 1942).
López Villalobos, Francisco: Diálogo sobre las fiebres interpoladas, Zaragoza, 1544.
Martín de Viciana, Rafael: Libro de Alabanças de las lenguas hebrea, griega, latina, castellana i valenciana, Valencia, Biblioteca Valenciana Digital, 1574.