I disagree that this shows the mutability of religion, JudeoChristianity in particular.

Consider: when Jesus came along, some people believed he was the messiah; others didn't. Those that did founded Christianism; those that didn't went right on believing what the Tora says. To whit, Judaism did not change as a religion when Jesus came, it merely spawned a new one.

Consider: further down the road, when certain books came in contention with modern ideas, the Protestants spawned from Roman Catholisms. Note that once again, the parent religion, Roman Catholism, did not change but merely created a child religion.

Thus IMO the authors contention that all relgions are in fact immutable. That after their creation events, there might be small adjustments to interpretation and small details, but the big dogma, no matter what it is at the time of that religions creation, remains inviolate. Furthermore, this is in sharp counterdistinction to science in which very little of the ideas set forth at any fields creation event are unchanged over time.

Examples of this include: Newton and Einstein; Darwin vs. Modern Synthesis; Bohr and DeBroglie. In each case the former was a creation event for a scientific principle and the latter is a mutation of this creation event. Note as well that in changing, each did not spawn a child science like religion spawns child religions. We don't consider Newton or Darwin to be an "alternative" to Einstein and the Modern Synthesis as one might say that Judaism or Protestantism are viable alternatives to Christianity.

Your point is taken in that even within a religious text we see evidence of a mutable god but it's not god we are talking, he/she/it can change all he/she/it wants... once. But once a religion sets it's roots, it rarely if ever move. It sways, and it may grow, but more often will bifurcate instead of actually change.