 In 1149 the Crusaders , who had conquered Gaza in 1100 , built a cathedral dedicated to John the Baptist atop the ruins of the church upon a decree by Baldwin III of Jerusalem . However , in William of Tyre 's descriptions of grand Crusader churches , it is not mentioned . Of the Great Mosque 's three aisles today , it is believed that portions of two of them had formed part of the Cathedral of John the Baptist . 
