What held feudal society together?

Feudal society was held together by mutual obligations, sometimes based on custom, and sometimes based on vows.

The serfs and their lords had obligations to one another, which were inherited. The serfs owed their lords labor and had a duty to stay on the land; in exchange, the lords had to provide the serfs with land to work, places to live, and protection.

The lords and their overlords or monarchs had obligations to one another, based on feudal vows. The lords gave the their superiors oaths of loyalty and gave support of whatever nature was required, usually military; the monarchs and higher lords gave the lords manors to live on and from which they could get their incomes.

All of these people gave support to the Church, which responded with support of its own, both spiritual and temporal.