The Igbo tribe is held together by traditional customs, religious ceremonies, social hierarchy, familial relationships, and a dependence on farming that ensures a successful harvest and healthy economy. Chinua Achebe sought to depict the complex and rich culture of the Igbo people, which had never before been authentically represented in western literature without racial or cultural bias. Achebe illustrates the social hierarchy by depicting the egwugwu, commenting on the importance of titles in Umuofia, portraying a variety of social interactions, and giving an in-depth look at the egalitarian justice system established by the Igbo villagers. The Igbo villagers' complex belief system is also described and Achebe sheds light on the elaborate ceremonies and customs practiced by the Igbo people. The same features that hold the community of Umuofia together are practiced throughout the world in a similar capacity. Achebe succeeded in portraying the rich, civilized culture of the Igbo people, which relied on religion, traditional customs, social hierarchy, and a justice system to hold it together.

Religious and social customs, along with familial responsibility and tradition all contribute to the community's cohesion in Things Fall Apart. What is important to understand is that what holds the community of Umuofia together are the very same things that hold together the communities of the missionaries...That is, Achebe wants us to see that the Umuofian society is intact; it has social traditions, a complex language, judicial systems, and a committment to family, not unlike any other organized and civilized community, regardless of what the colonizers might believe.

The link below describes how custom and tradition hold the community, and indeed the tribe, together - read especially the sections "Custom and Tradition" and "Culture Clash".

The community derives identity and stability through its traditions - roles are defined and accepted ways of relating to each other and the whole are clearly laid out for individuals.  When the white man comes, things begin to fall apart.  The "...tribe is split...the bond that ties the tribesmen deteriorates".