This Research Paper examines how the white supremacist movement Christian Identity emerged from a non-extremist forerunner known as British Israelism. By examining ideological shifts over the course of nearly a century, the paper seeks to identify key pivot points in the movement’s shift toward extremism and explain the process through which extremist ideologues construct and define in-group and out-group identities. Based on these findings, the paper proposes a new framework for analysing and understanding the behaviour and emergence of extremist groups. The proposed framework can be leveraged to design strategic counter-terrorism communications programmes using a linkage-based approach that deconstructs the process of extremist in-group and out-group definition. Future publications will continue this study, seeking to refine the framework and operationalise messaging recommendations.