Like Starz’s “American Gods” before it, Amazon’s newest limited series attempts to adapt the philosophically lofty, theologically-infused work of British novelist Neil Gaiman. In his 1990 novel “Good Omens” (co-written with Terry Pratchett), an odd-couple angel and demon attempt to avert the apocalypse in order to continue enjoying their unusual partnership. Onscreen, this pairing — between a saintly being played by Michael Sheen and a fallen angel played by David Tennant, both seeking to save the world for their own reasons — is the best part of the new “Good Omens” limited series. But it’s not enough: This six-hour journey towards the end of time comes to feel grindingly slow by the end, more anticlimax than fight for Earth’s future.

We meet Sheen’s Aziraphale and Tennant’s Crowley at the very beginning of recorded myth, in the Garden of Eden. Their personalities shake out with crystalline clarity (fitting...