Marduk's supremacy is argued, not merely asserted, in the Babylonian creation epic Enuma Elish ("When on high"). The older gods are threatened by Tiamat, the primordial saltwater ocean personified as a monster of chaos, and none can face her, until the young Marduk agrees, on one condition: that the gods grant him kingship over them all. They test his word, he proves it, and they hail him king. He then goes out, kills Tiamat, splits her body to form sky and earth, sets the stars in their courses, and organises the cosmos. Finally he creates humankind from the blood of a defeated rebel god, so that people may do the gods' labour and the gods may rest.
The epic ends by reciting Marduk's fifty names, each transferring to him a power or domain of an older god, a literary act of concentrating the whole pantheon's authority into one figure. It was recited at the Babylonian New Year (Akitu) festival, where the king's own legitimacy was renewed before Marduk.
Read as scholarship, Enuma Elish is a theological and political charter: it justifies Babylon's primacy and its king's rule by making Babylon's god the maker and manager of the world. Read as religion, it is the account of how order was won from chaos.