Note Williams doesn’t ask whether Jesus or the New Testament promote tribalism. They don’t. He is asking whether Christianity does.
The answer seems to be “yes”
Note, too, what Williams means by “tribalism”: not any claim to exclusivism, but
an exclusive mentality that leads to indignation, shunning, meanness, oppression, violence, and even murder.
The great irony of the history of Christianity is its own history of tribalism, given that Jesus and Paul both worked to break down “walls of hostility” (Ephesians 2:14) between groups in their time. We have the obvious, egregious, examples with the Crusades, Inquisition, and witch-hunts, but Williams wants us to think about more common examples that play off of insider-outsider thinking.
Philip Jenkins in “Jesus Wars” mentions an even greater bloodshed that occurred between Christians in the 5th and 6th centuries as they fought over doctrinal differences. Even today, many of us know pastors and professors who were fired because they came to accept modern science, or happened to disagree with some archaic point of doctrine formulated 400 years ago, or decided that women could become pastors.
The only way people can do this is through pretense—a form of self-deception. Pretend that men are superior to women, Serbs to Croats, Whites to Blacks, rich to poor, CEOs to workers, Americans to Iranians, Calvinists to . . . everyone else.
There is probably some spiritual law that the more exclusive the group, the more pretense is needed to establish and maintain the group, and thus the more self-righteous and immoral it becomes.
The question is how the gospel (not Christianity) can speak to tribalism even among its own.