This book is a comparative theological investigation into the following question: how does one theologically understand the sincere-truth-seeking religious other who rejects one's truth claims not out of animosity or ignorance, but rather from a desire to worship God in spirit and in truth? Specifically, this book investigates the extent to which soteriologically exclusivist Muslims and Christians maintain their respective truth claims while maintaining a posture of vulnerability to the revisionary power of the religious other's claims. To answer these questions, this book examines comparative theology's missiological foundation through a dialogical study of neo-Calvinist and Reformist Sunni understandings of the epistemic status of the religious other. This book is a practice in comparative theology with the goal of rethinking neo-Calvinist theology of religions through Islamic thought to present a missiological comparative theology amenable to exclusivist theological positions within Christianity and Islam.