Final answer:
Enns views the development of monotheism as the unifying theme of the Old Testament.
Step-by-step explanation:
The unifying theme of the Old Testament according to Enns is the development of monotheism. The sacred writings compiled during this period emphasized the exclusive worship of Yahweh, the God of the Hebrew people. Over time, the concept of Yahweh evolved from being a territorial god to becoming an omnipotent and omnipresent deity. This gradual development towards monotheism was influenced by historical events and the interaction of the Hebrews with other civilizations.