The area, where the present mosque of Damascus stands, was already a sacred place in Greek times as there was a temple to Zeus, which in Roman times was dedicated to Jupiter. During the Severan dynasty, following some modifications, it became one of the largest areas of worship in Syria. At the end of the 4th century, Emperor Theodosius ordered the pagan temple to be transformed into a Christian church dedicated to St John the Baptist. After the Arab conquest, it became a place of worship for Christians and Muslims from 661 onwards thanks to the building of a musalla, and in 706 A.D., with the Umayyad caliph a-Walid I, it was finally transformed into a mosque. The Great Baptist Church of Theodosius was incorporated into the new building, entirely decorated with marble and mosaics.