Paganism flourished in İstanbul before Christianity was established in the 4th century, when churches began to appear. The first churches in İstanbul were the Havariyun Church, Haghia Sophia and Haghia Irini. Many churches were built up to the time of the Ottoman Conquest and many sects were represented. i.e. Nestorians, Monophysites, Catholics, Orthodox, Assyrian, Gregorian, Dominican and Francisians. Also the Greek , Armenian, Latin and Genoese built their own churches.

After the conquest Sultan Mehmed, the Conqueror left the churches free to worship, but in the Ottoman Era many church buildings were sold and those which were derelict or abandoned were purchased and converted to mosques. The construction of churches nevertheless continued during the Ottoman Period but most were built outside the city walls in places such as Beyoğlu and summer resorts as well as along the shores of the Bosphorus. After the 19th century most new churches were located close to the embassies. The prominent ones are Saint Maria Draperis, St. Antoine de Padoue, Latin Italian Church, Armenian-Catholic Surp Yerrontutyan Church, Terre-Sainte Spanish and the British Embassy Church.

Similarly synagogues in İstanbul are as old as the churches. The first known synagogue was built in 318 A.D.. Synagogues managed to survive in spite of the fact that some were converted to churches from time to time or were pillaged during the Latin Occupation in the thirteenth century. The number of synagogues in İstanbul of Jews from Spain and other parts of Europe as the result of the Inquisition in the 15th century A.D., the number of synagogues increased even further.

Synagogues built during the Ottoman period did not have a distinctive architectural style. They all share a very non-decorative, simple appearance and were built in courtyards in a plain rectangular shape. Many synagogues constructed during the Ottoman period are still active and serving the Jewish community in İstanbul today.

SAINT ANTOINE CHURCH

Located in İstiklal Street in Beyoglu, the Saint Antoine Church is on the left side of the street if you are facing from Galatasaray towards Tunel. Construction began in 1906 and the church was completed in 1912. Its architect, Giulo Mongeri, who was born in İstanbul, gave it an Italian Neo-Gothic style. Today it is İstanbul's largest church with the busiest congregation and is run by Italian priests. The Church was built in a courtyard. The entrance of the church in on the main street between two apartments which were built to earn money for the church.

STEPHAN (BULGARIAN) CHURCH

This church belongs to the Bulgarian minority and is the most interesting church in İstanbul. The Bulgarian minority of the Ottoman Empire used to pray at the churches of the Fener Orthodox Patriarchy. Due to the nationalistic movements, Bulgarians were allowed to build their own church in the 19th century. First, a small wooden church was built on the shore of the Golden Horn between Balat and Fener squares where the current church is located, but later this was developed into a larger building. An iron frame was preferred to concrete reinforcement due to the weak ground conditions.The construction plans were prepared by Hovsep Aznavur, an Armenian of Istanbul origin. An international competition was conducted to produce the prefabricated parts of the church. An Austrian firm, R. Ph. Wagner, won the competition.

The prefabricated parts were produced in Vienna and transported to İstanbul by ship through the Danube and the Black Sea. After one and a half years work, it was completed in 1898. The main skeleton of the church was made of steel and covered by metal boards. All the metal pieces were attached together with nuts, bolts, rivets or welding. The architectural styles come from the Neo-Gothic and Neo-Baroque periods.

THE PATRIARCHATE OF GREEK

ORTHODOX CHURCH

Located in the Fener region, between Sadrazam Ali Pasha Street and Incebel Street, the Roman Othodox Partiarchate is located in the yard of this church. In 1602, the Patriarch moved to Aya Yorgi, when the site of used as monastery. Since that time, a good many renovations have occured, the most recent of which ended in 1991, owing to damage sustained by fire in 1941.

Although not so significant architecturally. Aya Yorgi possesses valuable historical artifacts. Among the most significant are: a patriarch's throne, dating circa the 5th century; three samples of rare mosaic icons; a column which is believed to have been used for the binding an flogging of Jesus in Jerusalem; and cascades belonging to three women saints.

SURP KRIKOR LUSAROVICH ARMENIAN

ORTHODOX CHURCH

Built in 1960 after its predecessor was demolished to widen Kemeraltı Street (between the Galata Tower and Bridge), Surp Krikor Lusarovich, an Armenian Orthodox Church, borrows its name and its striking white form from an original Seventh century church in Armenia. On view of the crypt are some elaborate tiles saved from the previous church on the site, which was otherwise a rather unremarkable structure.

ASSYRIAN CHURCH

The Church is on Karaburun Street in Tarlabasi, Beyoglu and is the only church in İstanbul to be built by Assyrian. The Assyrian generally use churhes that they either rent or borrow from the other denominations. The Virgin Mary Church was built in 1960 and its stones were brought from the Assyrian center in Mardin. There are also other sections in the church like a school and an administration office.

THE ANGLICAN CHURCH

Located in Beyoğlu at 83 Serdar-i Ekrem Street This church also known as the Crimean Church was designed by the British achitect G.E. Street.

The church land was donated by Sultan Abdulmecid and the church was dedicated to the memory of the British Soldiers who died in the Crimean War. Started in 1858 it was ten years before it was completed. Closed in 1978 due to the lack of congregation, it was reopened in 1991. The church was built of Maltese stone is of neo-gothic style.

NEVE SHALOM SYNAGOGUE

The synagogue is on the Buyuk Hendek Street in the Kuledibi district of Beyoglu square. The Neve Salom ("Peace Oasis") Synagogue was built by renovating the gymnasium was converted to a synagogue in 1938, but it couldn't be used because of a lack of permission. Permission was granted in 1949, and the project was prepared by Elio Ventuos and Bernard Motola, two graduates from Istanbul Technical University. It was opened for prayer and worship in 1951.

Its most striking attractions are the dome which holds an eight ton chandelier, the stained glass windows which were imported from England, and the marble sections.

The Synagogue was bombed by terrorists in 1986, but was renovated and reopened in 1987.