Border Lines is Daniel Boyarin’s groundbreaking exploration of how Judaism and Christianity—once intertwined—gradually defined themselves in opposition to one another. With scholarly precision and compelling insight, Boyarin traces how theological “boundaries” were drawn, not by natural separation, but through deliberate debates, interpretations, and identity-shaping choices. This book invites readers to rethink assumptions about early Christianity, rabbinic Judaism, and the shared world from which both emerged.