Nonetheless , the cult of Ishtar and Tammuz managed to survive in parts of Upper Mesopotamia .
Traditional Mesopotamian religion began to gradually decline between the third and fifth centuries AD as ethnic Assyrians converted to Christianity .
The cult of Ishtar and Tammuz managed to survive in parts of Upper Mesopotamia .
Traditional Mesopotamian religion began to gradually decline between the third and fifth centuries AD as ethnic Assyrians converted to Christianity .