I'm also reading The Christian Imagination: Theology and the Origins of Race by Willie James Jennings. I finished the first chapter yesterday.
That chapter begins with a narrative of the first African slaves returned to Portugal and sold, all this as part of the beginning of the colonial age with the adventures of Prince Henry the Navigator. Jennings engages in interesting theological analyses of the writings at the time which gave Christian justifications for these new actions. After that the chapter became very dense.
The thesis of the chapter is that prior to the modern age identified with the start of colonialism, human identity was largely shaped by the landscape in which one existed. He cites numerous examples from a wide range of cultures. What happened in colonialism was a displacement that changed how identity was structured. Land became something you conquered and possesed rather than being shaped by and in. He argues that the Christian doctrines of creation ex nihilo, incarnation, and of supersessionism paved the way for this new age. The prince or pope who embodied Christ could replace pre-existing cultures and peoples and create a new Christian environment.
It was after this change in how identity was structured occured, that race as an identity category was created. He does not complete this next step of the analysis in the first chapter.