On the One and Eternal Testament or Covenant of God

Heinrich Bullinger's treatise on God's single and eternal testament emerged from the heated theological debates of the 1530s, particularly disputes over the relationship between the Old and New Testaments that divided Reformed theologians from Anabaptists and others. Writing as Zwingli's successor in Zurich, Bullinger sought to articulate a coherent understanding of God's covenant that would counter both Anabaptist rejection of infant baptism and Lutheran approaches to law and gospel. The work represents one of the earliest systematic treatments of covenant theology in Reformed thought.

Bullinger argues that God has established one covenant of grace that spans both testaments, differing only in its modes of administration rather than in substance. He contends that the covenant made with Abraham is fundamentally identical to the new covenant in Christ, both offering salvation by grace through faith. The treatise carefully distinguishes between the eternal covenant of redemption within the Trinity and the historical covenant of grace revealed progressively to humanity. Bullinger emphasizes that circumcision and baptism serve as signs of the same covenant reality, thereby defending infant baptism against Anabaptist objections while maintaining the unity of Scripture against any suggestion that God operates by different principles in different eras.

This work profoundly influenced Reformed covenant theology for centuries, shaping how Protestant theologians understood the continuity between Israel and the church. Bullinger's framework provided theological grounding for both sacramental practice and biblical interpretation that became standard in Reformed confessions. Who should read this: scholars of Reformed theology and covenant theology will find this essential for understanding the historical development of these doctrines, while those seeking purely devotional or practical spiritual guidance should look elsewhere, as this remains a technical theological treatise requiring familiarity with sixteenth-century doctrinal debates.