The mosque holds a shrine which is said to contain the head of John the Baptist ( Yahya ) , honored as a prophet by Muslims and Christians alike . After the Arab conquest of Damascus in 634 , the mosque was built on the site of a Christian basilica dedicated to John the Baptist ( Yahya ) since the time of the Roman emperor Constantine I. The mosque holds a shrine which today may still contain the head of John the Baptist , honored as a prophet by both Christians and Muslims alike .