A in-depth exegetical study of a key biblical term by a preeminent Reformed theologian Covenant is key a concept in both the Old and New Testaments, playing a primary role in Israel's self-understanding as a nation.It is also a key term for Christian self-understanding, but the Greek word for covenant, diathéké , occurs relatively infrequently in the New Testament except in Hebrews where it draws on Old Testament usage to illustrate it's meaning for Christians.Vos examines occurrences in both the Old and New Testaments, bringing to light why the term is so difficult to understand while also fully explaining its meaning for Christians as a critical concept for how Christians can understand their relationship to God as a covenanted people.Important also for Reformed systematic theology, Vos' study provides a foundational examination that will help Christians understand the full message of Scripture more completely.