Religion in Bulgaria has been dominated by Christianity since its adoption as the state religion in 864. The dominant form of the religion is Eastern Orthodox Christianity within the fold of the Bulgarian Orthodox Church. During the Ottoman rule of the Balkans, Islam spread to the territories of Bulgaria, and it remains a significant minority today. The Catholic Church has roots in the country since the Middle Ages, and Protestantism arrived in the 19th century; both of them remain very small minorities. Today, a significant part of the Bulgarians are not religious, or believers who do not identify with any specific religion, and Bulgaria has been the cradle of some new religions, notably the Neo-Theosophical movement of Dunovism.

Since the early 21st century, there has been a decline of both historic religions of Bulgaria—Eastern Orthodox Christianity and Islam. Orthodox Christianity has shrunk from 7.3m or 87% of the population in the 1992 census to 4.4m or 60% in 2011 and 4.1m or 63% in 2021, and Islam from 1.1m or 13% in 1992 to 0.6m or 10% in 2021. Over the same timespan, Protestantism and other non-Eastern Orthodox and non-Catholic Christianities have grown from about twenty thousand or 0.3% of the population in 1992 to about eighty thousand or 1.3% in 2021, while unaffiliated people, comprising both not religious people and people who have some belief but not identifiable with any specific religion, have grown from 0.3m or 4% in 2001 to 1m or 16% in 2021. Until the 1992 census, Bulgarians were obliged to declare the historic religious belonging of their parents and/or ancestors, while since 2001 people were allowed to declare personal affiliation or unaffiliation to any religion. Moreover, beginning with the 2011 census, Bulgarians were allowed to avoid giving any answer to the question about religion; 22% of the population in 2011 and 9.5% in 2021 did not answer.

After the end of the socialist and one-party People's Republic of Bulgaria (1946–1990) and the transition of the country to a parliamentary republic, the revival of Islam was stronger than that of Eastern Orthodox Christianity, and the Bulgarian Orthodox Church has witnessed a serious decline in its membership from 2001 onwards. The authority of the Bulgarian Orthodox Church was undermined since the 1990s because of its collaboration with the erstwhile socialist regime, which was fully revealed with the opening of the state's secret archives in 2012, according to which eighty percent of the clergy were former members of the secret police. Articles 13 of the Constitution of Bulgaria designates Eastern Orthodox Christianity as the traditional religion of the country, but guarantees the free exercise of any religion, that religion is separate from the state, and that it shall not be used for political aims; Article 37 affirms that the freedom of choice of different religious or irreligious views is inviolable, and the state shall safeguard the harmony between them. Unlike former Yugoslavia in the 1990s, Bulgaria has not experienced any significant ethnic or religious strife, and the religious communities of the country coexist peacefully. In fact, the capital Sofia is known for its so-called "Square of Religious Tolerance", where the Church of Saint Nedelya, the Cathedral of Saint Joseph, the Banya Bashi Mosque and the Sofia Synagogue are located within close proximity from each other in the very centre of the city.

Census statistics 1887–2021: Raw data

The Bulgarian nation emerged in the Early Middle Ages (7th–10th century CE) as an amalgam of ethnic elements of different origins: Southern Slavs, who populated the Balkans from the 5th century; pre-Slavic native populations, the Thracians who had been Hellenised and Romanised during the previous foreign dominations; and Bulgars, a Turkic population who migrated from Central Asia to the Danube region and founded the First Bulgarian Empire (681–1018). The Bulgarian khans dominated over this heterogeneous population, whose components originally co-existed as separate communities with their own religious systems and traditions (Slavic paganism the Slavs, Thracian paganism the Thracians, and Tengrism the Bulgars). Christianity was adopted — in its Byzantine Orthodox form, then still part of a United Church, from the neighbouring Byzantine Empire — as an ideological and ethnic homogeniser around 864 by the khan Boris-Mihail; the Christianisation of Bulgaria was largely a political expedient which granted the Bulgarian khans the same status as other European monarchs, but it also met considerable opposition, especially from the aristocracy and even from Boris-Mihail's son and heir to the throne, Vladimir-Rasate, who tried to suppress Christianity and revert to paganism as he saw the former as a tool of political yoke from Byzantium.

In 870, the Christian hierarchs of Bulgaria took part in the Fourth Council of Constantinople, and the council granted the Bulgarian Orthodox Church the status of an autonomous archbishopric under the Patriarchate of Constantinople, from which it obtained its first primate, clergy and theological books. In 919, the Bulgarian knyeaz Simeon adopted the new title "tsar of the Bulgarians and the Romans" (tsar was an adapted form of the Latin title caesar; the change reflected an ideal acquisition of the Roman imperial tradition) and heightened the Bulgarian Orthodox Church to the status of autocephalous patriarchate, independent from the Patriarchate of Constantinople. Sixteen years before, in 893, an ecclesiastical council decided to switch to Old Church Slavonic (Old Bulgarian) as the liturgical language, and in the meanwhile the Cyrillic alphabet was developed for writing the Slavic language; such changes protected the Bulgarians from Hellenisation, and ultimately would have been fundamental for the later history of part of the Slavic peoples. In 927, the autocephaly of the Bulgarian Orthodox Church was recognised by the Patriarchate of Constantinople.

The emerging Bulgarian Orthodox Church retained many elements of pre-Christian paganisms. Moreover, since the 10th century Bulgarian Christianity was deeply characterised by the Gnostic doctrine of Bogomilism, developed in Bulgaria itself by the priest Bogomil, as well as by the ascetic doctrine of Hesychasm. Both Bogomilism and Hesychasm were highly spiritual, mystical and meditative doctrines and practices, favouring the inner (esoteric) path to God and organised around monasticism, but while the former was dualistic, with an accentuated distinction between spirit and matter, the latter was monistic and spread to Bulgaria largely as a reaction to the former.

In 1396, the disintegrating potentates of the Second Bulgarian Empire (1185–1396) were conquered by the Ottoman Empire, a Turkish empire whose official religion was Islam, specifically Sunnism. In Ottoman Bulgaria (1396–1878), like elsewhere in the Ottoman Empire, populations were classified according to the millet (approximately "religious nation") system by religion rather than by ethnicity, and therefore Bulgarian Orthodox Christians were grouped together with Orthodox Christians of other ethnicities in the so-called Rum Millet ("Roman Nation"), all under the jurisdiction of the Patriarchate of Constantinople — therefore, the Bulgarian Orthodox Church lost its autocephaly.