The Trinity is one of the biggest issues encountered when discussing the Christian faith with those who believe in God, but not quite the God of Christianity.  Mormons, Muslims and Jews all believe in the God of Abraham, Isaac, and Jacob; however, they believe in a strictly monotheistic God, one being and one person (’yachid’ in Hebrew meaning a solitary numeric one), while Christians are monotheists who believe in God who is three persons in one being (’echad’, a composite oneness in unity and essence).   In the course of the conversation, the yachidist[1] monotheist might say something along the lines of “The Jews of Jesus’ day had no concept of multiple persons in the Godhead.  The trinity is a later invention forced on the church by Constantine and incorporated beliefs of Babylonian/Egyptian/Greek/take-your-pick paganism”[2] Many people believe this, but it is not at all true. Two Powers in Heaven by Alan Segal explores and analyzes the early rabbinic commentary against those who believed there were “two powers” in heaven. Segal writes from a position that opposes Trinitarianism and that is considered orthodox within modern Judaism. He was a “yachidist,” a strict monotheist. Segal did not believe that there are “two powers” which are God and he considered the belief in “two powers” a heresy; however, he did recognize that this belief was found within Judaism in the first century and earlier. The book researches the first arguments against the “two powers heresy” and explores the possibilities for the targets, including Christians, Gnostics, and sects of Judaism. The Origins of Modern Judaism Before I review the actual book, I would like to give a brief history of the origins of modern Judaism. During the Second Temple period, there were a number of sects of Judaism.  Through reading the New Testament, we are familiar with two of those sects, the Pharisees and the Sadducees; however, there were other sects such as the Essenes and the Zealots.  Within those sects, there were also varying beliefs.  After the destruction of the Temple by Titus in 70 A.D.,  the Levitical system of worship and sacrifice came to an end and what became rabbinic Judaism formed out of the Pharisaical schools.  Rabbinic writings, such as the Mishnah and the Talmud, began to be codified at the end of the second century A.D[3]. as Segal notes, in the “Two Powers.”  Many of the earliest writings found in the Mishnah and Talmud are polemics written in response to Christian beliefs which the rabbis rejected. An Overview of the Two Powers in Heaven Segal begins by giving an overview of the rabbinic discussion regarding the “two powers” heresy and presents possibilities for their object.   Just as we today have a plurality of beliefs about God and his nature, there were diverse beliefs at the time of the second century.  Beyond the Christians who believed in three persons of the Godhead, Gnostics were also prevalent.  The most common form of Gnosticism was dualistic in nature believing in two opposing deities, one good and one evil; however, there also appears to have been atheists who believed in “no power in heaven.[4]”  Regardless, the “two powers” polemic was directed towards all those who were not monotheists,[5] and more specifically, those who did not believe in a God with a single person, one who was “yachid.” In part two, Segal lays out the evidence of the issue itself and explains why it was so hotly debated. He includes references in Scripture of what he considers to be “conflicting” appearances of God along with rabbinic interpretation. A discussion on the nature and purpose of angelic powers is also covered. The final section discusses the differing views at the time of the beginning of the new millennia. Segal examines how philosophers such as Philo (more on this later) viewed the text as well as apocalyptic sects such as the Essenes. Segal also presents the case of the early church writers and the Gnostic view.