In this volume, Pohle traces references to the doctrine of the Trinity in the Old and New Testaments, and shows how the Trinity is foreshadowed in God's covenant with Israel and in the fulfillment of the covenant in Christ. He then outlines a detailed doctrine of the Trinity, drawing from the entire Bible, the liturgies of the Early Church, and the subsequent development of doctrine. A discussion of various heresies in the church's history also figures prominently.The second half of this volume deals with the relationships between each person within the Trinity, along with the ways in which the Trinity is revealed--through reason, revelation, and the acceptance of mystery. Pohle asserts that evidence for the Trinity exists in both reason and revelation, but famously states: "Christians must first believe, then inquire."