Bulgaria is officially a secular nation and the Constitution guarantees the free exercise of religion but designates Orthodoxy as a "traditional" religion. In the 2001 census, 82.6% Bulgarians declared themselves Orthodox Christians, 12.2% Muslim, 1.2% other Christian denominations, 4% other religions  and zero percent atheists. Most citizens of Bulgaria have associations — at least nominally — with the Bulgarian Orthodox Church. Founded in 870 AD under the Patriarchate of Constantinople , the Bulgarian Orthodox Church had autocephalous status since 927 AD. The Church became subordinate within the Patriarchate of Constantinople, twice during the periods of Byzantine  and Ottoman  domination. It was re-established first in 1870 in the form of the Bulgarian Exarchate, and then in the 1950s as the Bulgarian Patriarchate.
Question: What is this paragraph mainly about?
Answer: Bulgarian Orthodox Church