According to the Bulgarian census of 2001 about 6.5 million Bulgarians are Orthodox Christians or 83% of the populations, 12% are Muslims, .6% Roman Catholics, .6% Protestants, and the rest practice other religions or do not have any religion at all. Though the Constitution grants religious freedom, it forbids unregistered groups practicing their religion in the public.
The principal religion in the country is undoubtedly the Bulgarian Orthodox Church. Other Orthodox churches include the Greek Orthodox Church, Romanian Orthodox Church, Russian Orthodox Church, and Ukrainian Orthodox Church. Christianity began in the country during the First Bulgarian Empire in mid-9th century when Boris I rules the kingdom. The Bulgarian Orthodox Church is the oldest Slavic Orthodox Church and has provided significance influences in the Middle Ages. Islam is the second largest religious group with approximately 1 million followers as of 2001. It arrived in Bulgaria during the Ottoman Turkish attack of the Balkans between 14th and 15th century. Most Muslims reside in the Kardzhali Province, Razgrad Province (mostly Turks), and Smolyan Province.
Roman Catholicism was spread by the Bulgarianized Saxon ore miners in the northwestern region of the country as well as by Paulician and Bogomil missionaries. Many Catholics can be found in Rakovski in the Plovdiv Province and in some communities in northern Bulgaria. Protestantism came in the country during the 19th century through American missionaries. Other religions are the Armenian Apostolic Church, mostly living in Plovdiv, Sofia, Burgas, and Varna and Judaism which also had important cultural influences over the past decades.