The sign of the fish is one of the most frequent images used by the early Christians. The Greek word in the center is an acrostic that stands for the initial five letters of the initials of Jesus/Christ/of God/the Son/Saviour. The acrostic was in use by the 4th century AD, if not earlier. Tradition records that during the early years of persecution, the sign of the fish was used by Christians to identify each other. This cross was taken from an image in Calixtus catacombs.