Christianity was the official religion of the Roman Empire and the Eastern Roman Empire, and the dominant faith in Constantinople for most of its history. There were various sects and schisms within Christianity, such as the Chalcedonian, the Monophysite, the Nestorian, the Orthodox, the Catholic, and the Protestant.
Islam was the official religion of the Ottoman Empire, which conquered Constantinople in 1453 and ruled it until 1922. Many Muslims migrated to the city or converted from other religions, creating a diverse and influential Muslim community. There were also different sects and schools of Islam, such as the Sunni, the Shia, the Sufi, the Hanafi, and the Shafi'i.
Judaism was a minority religion in Constantinople, but had a long and rich history. Jews had settled in the city since ancient times, and faced various persecutions and protections under different rulers. They contributed to the culture, trade, and scholarship of the city, and formed different communities, such as the Romaniote, the Sephardi, the Ashkenazi, and the Karaite.
Paganism was the original religion of the ancient Greeks and Romans, who founded and named the city of Constantinople. It included a polytheistic worship of gods and goddesses, as well as various rites and rituals. Although it was largely suppressed and replaced by Christianity, some traces of paganism survived in Constantinople, especially among the rural and lower classes, in folk customs and superstitions, and in some secret cults and sects.