Once we moved past small family groups, we needed something else to unite us in a larger social group. Ideas and beliefs are it.
This is an interesting thought. In ancient Mesopotamia, each city-state had a patron god by which the city was known. Syria was previously called Assyria, named for its capital city Assur, which was also the name of its main god (Ashur). Wars between city-states were mythologised as conflicts between their patron deities. To some degree, this tradition extended to Greece and Rome.
More nomadic civilisations don't seem to have defined themselves by their gods, though they had them. Genghis Khan, for instance, observed his religion and consulted his shamans before actions, but the unifying factor among Mongol tribes was that they were Mongols by culture and by bloodlines. They didn't think of themselves as fighting for their gods, but for Mongols and their supreme khan.
I think you would have seen similar among Northern & Western European peoples, too - they were united by what they saw as their nationality: Goths, Jutes, Angles, Gauls and so on. Like the Mongols, they were loyal to both their tribal leaders and an overarching leader whose call they would answer at any time.
I agree that we still seek belonging and leadership.