religion, Attic began to be used in Christian writings in addition to and often interspersed with koine Greek. Nonetheless, from the 6th at least until the 12th century, Attic remained entrenched in the educational system; while further changes to the spoken language can be postulated for the early and middle Byzantine periods. The population of the Byzantine Empire, at least in its early stages, had a variety of mother tongues including Greek. These included Latin, Aramaic, Coptic, and Caucasian languages, while Cyril Mango also cites evidence for bilingualism in the south and southeast. These influences, as well as an influx of people of Arabic, Celtic, Germanic, Turkic, and Slavic backgrounds, supplied medieval Greek with many loanwords that have survived in the modern Greek language. From the 11th century onward, there was also a steady rise in the literary use of the vernacular. Following the Fourth Crusade, there was increased contact with the West; and the lingua franca of commerce became Italian. In the areas of the Crusader kingdoms a classical education (Greek: παιδεία, paideia) ceased to be a sine qua non of social status, leading to the rise of the vernacular. From this era many beautiful works in the vernacular, often written by people deeply steeped in classical education, are attested. A famous example is the four Ptochoprodromic poems attributed to Theodoros Prodromos. From the 13th to the 15th centuries, the last centuries of the Empire, there arose several works, including laments, fables, romances, and chronicles, written outside Constantinople, which until then had been the seat of most literature, in an idiom termed by scholars as "Byzantine Koine". However, the diglossia of the Greek-speaking world, which had already started in ancient Greece, continued under Ottoman rule and persisted in the modern Greek state until 1976, although Koine Greek remains the official language of the Greek Orthodox Church. As shown in the poems of Ptochoprodromos, an early stage of modern Greek had already been shaped by the 12th century and possibly earlier. Vernacular Greek continued to be known as "Romaic" ("Roman") until the 20th century. Religion At the time of Constantine the Great (r. 306–337), barely 10% of the Roman Empire's population were Christians, with most of them being urban population and generally found in the eastern part of the Roman Empire. The majority of people still honoured the old gods in the public Roman way of religio. As Christianity became