The argument that monotheism (that is, the belief in the existence of one god) is exclusive and therefore lends itself to hatred of the ‘outsider’ is a persistent one.
In fact, Regina Schwartz, Professor of English and Religion at Northwestern University wrote a book about it: The Curse of Cain: The Violent Legacy of Monotheism.
“Monotheism,” she writes, “is not only a myth of oneness, but a doctrine of possession, of a people by God, of a land by a people, of women by men.” And with possession comes violence, she argues: “Whether as singleness (this God against the others) or totality (this is all the God there is), monotheism abhors, reviles, rejects, and ejects whatever it defines as outside its compass.”
French philosopher Michel Onfrey filed a similar thought in his later book Atheist Manifesto, writing “The Jews invented monotheism and everything that went with it. First divine right and its mandatory correlative: the chosen people exalted; other peoples discounted; a logical enough sequence. Then, more importantly, came the divine strength needed to buttress this heaven-sent right, because the sword arm is what guarantees its realization here below.”
Are Schwartz and Onfrey correct? John Dickson says “At one level, yes.” But that’s not the whole story.
In his book Bullies and Saints: An honest look at the good and evil of Christian history, Dickson quotes Walter Brueggemann, an American Old Testament scholar who accepts that the concept of monotheism can lead to cultural and geographical monopoly and even points to parts of the Old Testament that invite the charge of monopolistic violence on the part of believers toward unbelievers (for example, the Canaan genocide described in the Book of Joshua).
“My impression, however,” writes Brueggeman, “is that Schwartz operates with a deeply reductionistic view of monotheism, a flat and one-dimensional offer that is scarcely faithful to how God is given in the text.”
Brueggeman notes that, whatever we may make of particular violent passages in the Old Testament, there is an equally logical interpretation of monotheism that points in precisely the opposite direction, and which has a substantial claim to being the true trajectory of the Tanakh’s presentation of the God of Israel. A single, universal Creator may be seen as a being of infinite “plenitude”, of a generosity that has no need to “fight for turf”, since all turf comes from God, belongs to God, and is the object of God’s care. Schwartz herself notes this possibility, but does not pursue it. Brueggeman asks why not?
More recently, the celebrated Yale philosopher-theologian Miroslav Volf wrote an important essay on religion and violence, in which he asks: Why would monotheism not, in fact, bind humans together rather than divide them? After all, if one God is the source of all things, human beings suddenly become family.
“If one accepts the belief in one God,” Volf explains, “in an important sense everybody is ‘in’, and everybody is ‘in’ precisely on the same terms.” The concept of the “image of God” in humanity is not far away from Volf’s analysis. One Father regards us as his children, and calls on us to act toward one another as siblings. Believing in multiple gods, on the other hand, may well inspire tribalism and violence between worshippers of different gods.
“In a polytheistic context,” Volf notes, “violence may reassert itself with even more force, because it will necessarily be justified by locally legitimized or arbitrary preferences, against which, in the absence of a divinity which overarches the parties, there now can be no higher court of appeal.”
Dickson argues that the upshot of all this is “that there is no necessary connection between monotheism and violence.”
“The brutal warfare of the crusader and the lavish care of Mother Teresa can both be said to draw inspiration from belief in one God—just as a man’s love for a woman may lead to jealous fury, on the one hand, or to acts of heroic self-sacrifice, on the other.”
 Regina M. Schwartz, The Curse of Cain: The Violent Legacy of Monotheism. University of Chicago Press, 1997, 63.
 Michel Onfray, Atheist Manifesto: The Case Against Judaism, Christianity, and Islam. Arcade Publishing, 2005, 178.
 Walter Brueggeman, “Review of The Curse of Cain: The Violent Legacy of Monotheism”, in Theology Today 54.4 (January 1998), 534-37.
 Miroslav Volf, A Public Faith: How Followers of Christ Should Serve the Common Good. Brazos Press, 2011, 42.