During the Middle Ages, Christianity was the central element of Georgian culture. The development of a written Georgian culture was made possible by the creation of the Georgian alphabet for evangelization purposes. Monasticism played a major role in the following cultural transformation. It started in Georgia in the 6th century, when Assyrian ascetic monks, known as the Thirteen Assyrian Fathers, settled in Iberia and founded a series of monasteries, most notably David Gareja. They were soon joined by local monks, which led to the creation of significant works of hagiographic literature in Georgian, such as the Life of Saint Nino and the Martyrdom of the Holy Queen Shushanik. The golden age of Georgian monasticism lasted from the 9th to the 11th century. During that period, Georgian monasteries were founded outside the country, most notably on Mount Sinai, Mount Athos (the Iviron monastery, where the Theotokos Iverskaya icon is still located), and in Palestine. The most prominent figure in the history of Georgian monasticism is judged to be Gregory of Khandzta (759-861), who founded numerous communities in Tao-Klarjeti.