In a 2010 retrospective review Grognardia says Cults of Prax is "A true classic of the early days of the hobby. ...quite rightly considered one of the best treatments of religion in a fantasy RPG ever written and it's certainly one of the most inspirational."
Cults of Prax was published forty years ago this year. Its groundbreaking presentation of gods and how they interact with the world through those who worship them still makes it one of the most influential and important works ever released for RuneQuest and the world of Glorantha, and indeed for roleplaying games in general.
Indeed, Grognardia draws a comparison to Dungeons & Dragons' Deities and Demigods, released about the same time: Deities and Demigods "managed to reduce real world religious/mythological figures to mere game constructs; Cults of Prax does the opposite, elevating game constructs to something approaching credible, if clearly fantastical, belief systems."
Reviews from R’lyeh's own retrospective review (August 2019) says Cults of Prax is "arguably the most important supplement ever for both Glorantha and RuneQuest." The supplement "brought both function and form to faith, and in doing so, it made faith both playable and something that you wanted to play."
Reviews from R'lyeh also notes what reviewers at the time of the book's original release (1979) thought of it:
"I cannot rate this book too highly, it makes an already excellent, imaginative, and highly playable FRP system into a masterpiece that richly deserves a place at the forefront of the hobby. 10/10."—White Dwarf #23 (quoted in Reviews from R’lyeh review).
"[T]he best extant cosmology designed for use with any FRP that has been published."—Different Worlds #7 (quoted in Reviews from R’lyeh review).
"If you play RuneQuest, you want this book. If you are a serious Game Master in any fantasy system, you would do well to look it over… And remember: Gods don't have to be effective to be important. Belief is the thing, and the interactions of social groups and differing beliefs in Cults of Prax is good fantasy reading if you don't game at all."—Steve Jackson, Space Gamer #27 (quoted in Reviews from R’lyeh review).
CULTS OF PRAX is available in PDF and is easily compatible with the new RuneQuest: Roleplaying in Glorantha:
—from Chaosium: https://www.chaosium.com/cults-of-prax-pdf
—from DriveThruRPG: http://bit.ly/2zxGqDr