As Carlos rightly says, Easter celebrations are really worth watching as an important cultural particularity of this country. Many foreigners still believe that Spain, being a "catholic" country (and all those repeated stories about the Spanish Inquisition, the only one with formal courts and that killed infinitely less persons than any of the Dutch, German and French Inquisitions, for example), is dominated by religion. In the past years, and mainly from the 80s onwards, the decline of the religiosity of the Spaniards has been constant and steady.
According to Wikipedia (OK, probably not the most accurate source of info...but in this case they use actual sources from Jan 2022), QUOTE According to the Spanish Center for Sociological Research, 57.6% of Spanish citizens self-identify as Catholics, (38.7% define themselves as not practising, while 18.9% as practising), 2.8% as followers of other faiths (including Islam, Protestant Christianity, Buddhism etc.), and 38.1% identify as atheists (13.9%), agnostics (10.6%) or non-believers (13.6%) as of January 2022.
Most Spaniards do not participate regularly in weekly religious worship. A July 2021 study shows that of the Spaniards who identify themselves as religious, 36% never attend mass, 20.8% barely ever attend mass, 19% attend mass a few times a year, 6.8% two or three times per month, 13.4% every Sunday and holidays, and 2.9% multiple times per week. According to a 2021 survey, those who go to church several times a year are 17.3% of the total population; those who go several times a month, 9.3%; those who go every Sunday and all holy days of obligation, 14.9%; and those who go several times a week, 4.3%
Young people´s religiosity is in very low figures, less than 10%. But, as the article says, we love our traditions!! Source, https://en.wikipedia.org/wiki/Religion_in_Spain#:~:text=According%20to%20the%20Spanish%20Center,identify%20as%20atheists%20(13.9%25)%2C
I´d definitely try to enjoy this cultural diversity and attend any of the popular festivals and traditions during Holy Week in Spain, even if you´re not religious you´ll have a great experience.