Philo of Alexandria lists several examples of things that human beings will never know, one of which is whether the moon shines of its own light or merely reflects the light of the sun. Subsequent history has proven conclusively that he was mistaken. The lesson that I draw from this is that both a certainty of knowledge and a certainty of ignorance are equally precarious. We have seen how the meaning of “God is one” is fundamentally implicated with the question of knowledge of God, but I will not here hazard a conclusion regarding whether knowledge of God is easy, difficult, possible, impossible, or impious as our various Jewish thinkers have argued. And yet our history ought to be able to provide some conclusion about what “God is one” means in a Jewish context.

In this chapter, I argue that the expression “God is one” in Greek emerges later than some of the current scholarship supposes. I suggest that the expression is first attested, not among the Presocratic philosophers, but in Hellenistic Jewish pseudepigrapha. But the idea finds its first clear theological explication in the works of Philo of Alexandria. Yet speculations about divine unity and transcendence immediately led Philo into a philosophical quandary that has not been resolved two thousand years later. How can a God who is one and transcendent interact with a material world that is immanent and diverse? Philo endeavors to solve this problem by means of divine agents that he refers to as “Words” and “Powers.” But in so doing, he creates the potentially more serious problem of the relationship of the highest of these divinities with God, a problem that continued to trouble Jewish thinkers throughout history.

In the first part of this book, we will examine texts predicating the numeral “one” of nouns for God, gods, or the divine. I will demonstrate that, although the idea that the traditional deity of one people was the only God among its region’s pantheon that exists had developed by the middle of the first millennium BCE, expressing this idea by stating that “God is one” does not emerge before the second century BCE. We will begin in this chapter by examining “God is one” from its earliest beginnings: first in ancient Egypt, where third-millennium inscriptions praise the Egyptian sun god, “Greetings, sole one who constantly endures every day”; and then in the rise of YHWH as the proper name of Israel’s God, of whom the Bible declares, “YHWH is one.” Yet these early texts intend only to praise these deities, not to deny the existence of others.

In the Bible, “YHWH is one” meant to praise YHWH, not to deny that other gods exist. In Jewish Platonism, “God is one” meant not that there are no other divine beings but that these are at best secondary to God. To some Christian trinitarians, “God is one,” meant that there is one Godhead that is, even so, internally diverse. To the rabbis, “God is one” meant that there is an unbridgeable gap between God and all other beings. Medieval Islamic and Jewish philosophers constructed yet another conceptualization: “God is one” means that God is somehow ontologically one. This in turn led to “negative theology”: that God could not be described through positive attributes that suggest diversity, but only though negative predicates. This innovation led to troubling questions. If “God is one” means that God’s unity is entirely unlike the diversity of everything else, how is knowledge of God possible?

This chapter, which begins the second part of this book examining the range of meanings that “God is one” can bear, is concerned with the rabbinic literature. In contrast to Philo’s frequent insistence that “God is one,” the rabbis rarely express this sentiment as plainly, although they were even more strongly committed to it. Rather, they mostly express ideas about God’s unity indirectly: negatively by polemicizing against “two powers” or obliquely by citing Deuteronomy 6:4. In this chapter, I argue that the rabbis refrained from explications about “God is one” because their concern was not the numerical unity or diversity of God but responding to beliefs in divine agents or intermediaries, even subordinate creations whose existence need not necessarily conflict with belief in one God. In other words, the targets of rabbinic polemics were Jews who believed in the Word or Wisdom not Christian trinitarians as many scholars suppose.

Medieval Jewish negative theology and its obscure notions of divine attributes spurred a reaction among some Jews who wanted a more manifest sense of knowledge of God. This reaction was one of the factors that led to the emergence of the medieval Jewish mysticism known as the Kabbalah. The Kabbalists accepted the philosophers’ negative theology, but they wanted to know what they were negating. They accomplished this unaccomplishable paradox through the ten Sefirot. The Sefirot are a type of divine creative energy that facilitates God’s interactions with the world; they are the channels through which the divine transcendence achieves a progressively manifest immanence. Although God in essence in unknowable, the Sefirot are a knowable reflection of divinity. For these Kabbalists, “God is one” meant that God as God truly is one. Yet God from the human perspective is not one but ten.

“God is one” has been called Judaism’s “primary testimony of faith.” In this book, I examine what these specific words, taken as a kind of religious slogan, mean in this Jewish context. Readers will likely already understand, of course, that these words do not have a single meaning and probably never did. The history that this book will narrate is about the surprisingly many meanings they can bear and how they come to take on some of these meanings at various periods. I will concentrate on important turning points in the history of Judaism and its intellectual progenitors and interlocutors – ancient Near Eastern religiosity and its Egyptian, Israelite, and Judahite varieties; Hellenistic and rabbinic Judaism and early Christianity; medieval Jewish and Islamic philosophy and early Kabbalah; and modern Jewish thought – to bring a long historical perspective to this question. The structure and tone of this book is therefore as a narrative history from antiquity to modernity.

Although the Hebrew Bible hardly uses the expression “God is one” at all, the idea that YHWH is the only God that exists does appear in its latest editorial layers. This chapter surveys how the Bible expresses YHWH’s relationship to other gods. We concentrate on three strands throughout the biblical books: first, an earliest stratum that preserves evidence of a theology according to which YHWH was one deity in a pantheon; second, the dominant biblical theology according to which YHWH was the greatest among the many gods in existence; and, finally, the theology that post-dates a deuteronomistic editorial hand, according to which only YHWH is God. This survey shows that the later ubiquity of the expression “God is one” was not inevitable. Yet verses declaring “YHWH is one” and “I am who I am” become the scriptural foundation on which later thinkers base a theology of God’s unity and transcendence.

The modern period saw innovations in Europe that lead directly to contemporary ideas about “God is one,” this chapter’s subject. From an intellectual-historical perspective, modernity is in surprisingly large part a matter of by what methodology and by whose authority the truth or falsity of various types of propositions – moral, philosophical, scientific, or theological – ought be judged. The major trend was that other sources of authority, importantly critical reason, had primacy over tradition. Yet “God is one” remained a stubbornly persistent notion. Even as radical a thinker as Spinoza was committed to it. A more traditionalist modern Jewish perspective sees “God is one” as a “slogan of faith” with no content, not about God but about the religious practitioner’s intentions. Knowledge of God is impossible, but an intention to serve God, even if always beyond human ability to realize, is possible. This aspiration is expressed when declaring, “God is one.”