This is a topic that I’ve long thought about. How do we define “Judaism”? This actually breaks up into two related clusters of questions. The first is whether, and how, Jews themselves have defined “Judaism,” whether they use that term (attested relatively late) or not. The second is how outsiders – including academics – define “Judaism,” and the extent to which those definitions due useful work. The former is known as an emic definition, the latter as an etic one.
My own work has been more on the etic side. How can scholars create a non-normative definition of “Judaism” that is useful to our work? I discussed this both in my book, Creating Judaism: History, Tradition, Practice and in an essay I published in the Journal of the American Academy of Religion. While I actually prefer to avoid using the word “Judaism” in most of my own scholarship, I do think that when carefully defined it can be useful for scholars. In those works, I develop a simply map for charting the different manifestations of what we might call “Judaism.”
All of this is a long-winded way of saying that I have new article on this topic, called “Defining Judaism: The Case of Philo,” in a new, excellent collection edited by Nickolas P. Roubekas, Theorizing “Religion” in Antiquity (Equinox, 2019), pp. 245-264. In this article I use the map that I previously developed to chart the contours of Philo’s “Judaism.”