Roman Catholicism, which has a long history in Spain, remains the dominant religion. Although it no longer has official status by law, in all public schools in Spain students have to choose either a religion or ethics class. Catholicism is the religion most commonly taught, although the teaching of Islam, Judaism, and evangelical Christianity is also recognised in law. According to a 2020 study by the Spanish Centre for Sociological Research, %  about 61 of Spaniards self-identify as Catholics, 3% other faiths, and about 35% identify with no religion. Most Spaniards do not participate regularly in religious services. A 2019 study shows that of the Spaniards who identify themselves as religious, 62% hardly ever or never go to church, 16% go to church some times a year, 7% some time per month and 13% every Sunday or multiple times per week. Recent polls and surveys suggest that around 30% of the Spanish population is irreligious.

The Spanish constitution enshrines secularism in governance, as well as freedom of religion or belief for all, saying that no religion should have a “state character,” while allowing for the state to “cooperate” with religious groups.

There have been four Spanish Popes. Damasus I, Calixtus III, Alexander VI and Benedict XIII. Spanish mysticism provided an important intellectual resource against Protestantism with Carmelites like Teresa of Ávila, a reformist nun and John of the Cross, a priest, taking the lead in their reform movement. Later, they became Doctors of the Church. The Society of Jesus was co-founded by Ignatius of Loyola, whose Spiritual Exercises and movement led to the establishment of hundreds of colleges and universities in the world, including 28 in the United States alone. The Society’s co-founder, Francis Xavier, was a missionary who reached India and later Japan. In the 1960s, Jesuits Pedro Arrupe and Ignacio Ellacuría supported the movement of Liberation Theology.

Protestant churches have about 1,200,000 members. There are about 105,000 Jehovah’s Witnesses. The Church of Jesus Christ of Latter-day Saints has approximately 46,000 adherents in 133 congregations in all regions of the country and has a temple in the Moratalaz District of Madrid.

A study made by the Union of Islamic Communities of Spain demonstrated that there were more than 2,100,000 inhabitants of Muslim background living in Spain as of 2019, accounting for 4–5% of the total population of Spain. The vast majority was composed of immigrants and descendants originating from the Maghreb (especially Morocco) and other number of Muslims, Buddhists, Sikhs and Hindus. After the Reconquista in 1492, Muslims did not live in Spain for African countries. More than 879,000 (42%) of them had Spanish nationality.

The recent waves of immigration have also led to an increasing centuries. Their ranks have since been bolstered by recent immigration, especially from Morocco and Algeria.

Judaism was practically non-existent in Spain from the 1492 expulsion until the 19th century, when Jews were again permitted to enter the country. Currently there are around 62,000 Jews in Spain, or 0.14% of the total population. Most are arrivals in the past century, while some are descendants of earlier Spanish Jews. Approximately 80,000 Jews are thought to have lived in Spain prior to its expulsion. However the Jewish Encyclopedia states the number over 800,000 to be too large and 235,000 as too small: 165,000 is given as expelled as possibly too small in favour of 200,000, and the numbers of converts after the 1391 pogroms as less. Other sources suggest 200,000 converts mostly after the pogroms of 1391 and upwards of 100,000 expelled. Descendants of these Sephardic Jews expelled in 1492 are given Spanish nationality if they request it.