Here's what the cover of The Only True God: Early Christian Monotheism in Its Jewish Context is going to look like:
First, any attempt to provide an image connected with ancient monotheism and Christology may be off-putting to some potential readers. If the book featured an image of Jesus, or of the Trinity, such a depiction would, when placed together with the title, seem to end dialogue. It would suggest that the one(s) depicted on the cover are "the only true God", end of conversation, and that is not the sort of approach taken in the book.
Second, most monotheistic traditions agree that the one God ultimately transcends the ability of any image or even any language to describe or do justice to. And so those mysterious lights symbolize that aspect nicely.