Professor C. H. Dodd in The Apostolic Preaching and its Development has drawn attention to the importance of the “Kerygma” as a more or less formal statement of the Christian Gospel, lying behind the earliest Christian documents; it determines the structure of Mark, the speeches in Acts and the form of the Creeds. This conventional form seems to be derived from Judaism. A statement of the mighty works of God in creation and the history of mankind, more especially in the deliverance of his people from Egypt, is a recurring feature of Jewish literature in the post-exilic era; perhaps the most notable example is Ps. 136; cf. Nehemiah 9. 6 ff., Is. 63. 7 ff. Such a recital of the mighty works of God in delivering His people was obviously necessary if the religion of Israel was to be changed from a primitive Semitic tribal cult into a system of monotheism, centred on the national deity, identified with the one God of Heaven; it was specially necessary to find an aetiological myth for the principal festival of the Passover. The emphasis on this deliverance reflects the process by which vague legends of a migration from Egypt have been conflated with the return from Babylon to form the biblical picture of the Exodus.