I am a Spaniard. I will list them in order from largest to smallest.
1) Roman Catholicism
2)Islam
3)Atheisisim
4)Judiasim
Spain is 94% Roman Catholic. All the other religions only total 6%.    
All religions are allowed in Spain, although the Roman Catholic Church is, by far, the most popular.
70% of all Spaniards are Roman Catholic which is a denomination of Christianity. 2% other religions,25% no religion
   
Spain.
70% of Spain is Roman Catholic, 2% other religions,25% no religion. Roman Catholocism is a denomination of Christianity
The main religion of Spain is Catholicism.Well, the main religion in Spain is Catholicism. 94% of the population is Catholic, but they do have other religions there too.
Muslims, Christians (who were all Catholics at this point) and Jews, in order of population numbers.
Roman Catholicism makes up a large percentage of the religions practiced in Spain. However, many do not practice religious worship or attend church.
About 75% of Spain is Roman Catholic. About 20% say they are atheists or non-believers.
Almost all ethnic Spaniards are either Roman Catholic or Atheist/Agnostic. However, Spain has small minorities of Muslims, Protestant Christians, Orthodox Christians, Jews, Hindus, Buddhists, Neo-Pagans, and Taoists. The presence of these minority religions is mostly a result of immigration.
Modern Spain has residents representing all world religions. Spain by majority is reported as Roman Catholic, although the country is a secular democracy.