“Monotheism” sounds like such a simple idea. What could possibly be simpler than having only one of something? But if there is anything that is not straightforward in the study of religion these days, it is the notion of “monotheism.” In connection with Judaism, there are discussions of when monotheism (or something quite like it) first appeared in Israel, if indeed it ever did appear, and whether it involved belief that only one being called “God” existed, or just the exclusive worship of only one God.

When one turns one’s attention to Christianity, many would assume that at least here, one has left the realm of strict monotheism behind, because of the doctrine of the Trinity. Yet, in studying the New Testament, I found that the more I challenged the assumption that I was witnessing a “parting of the ways” over the issue of monotheism, the more the texts made sense.

The argument of The Only True God, in a nutshell, is that there was a focus on one supreme God as solely worthy of worship, which most Jews and most non-Jews in the first century agreed was distinctive of Judaism. The Christian texts included in the New Testament fit within and presuppose that same understanding of God. The oneness of God only became a point of contention between Jews and Christians in later times, after new philosophical questions began to be asked, and after significant steps had been taken along trajectories that did indeed eventually diverge, but in the first century still overlapped.

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James F. McGrath is an associate professor of religion at Butler University and the author of the new book The Only True God: Early Christian Monotheism in Its Jewish Context.