The album cover was intended to feature a picture of an Auguste Rodin sculpture from 1889 – also called "The Eternal Idol"; however, the album cover actually features two naked models covered in paint and positioned to resemble the piece of art because permission to photograph the actual sculpture could not be secured. The paint had toxic effects and the models had to go straight to hospital at the end of the photo shoot.
Despite the sleeve credits, which cites Dave Spitz as bass player, all bass parts were completed by Bob Daisley, and Eric Singer completed all drum parts. The percussion credit to Bev Bevan relates to a few cymbal overdubs on "Scarlet Pimpernel".
The album was originally to be recorded with Spitz and vocalist Ray Gillen. The former was replaced by bassist/lyricist Bob Daisley during the initial sessions on Montserrat with producer Jeff Glixman, and the latter quit shortly after their return in England. He later joined the band Blue Murder. Tony Martin was hired and reconstructed the vocals under the guidance of Chris Tsangarides at Battery Studios shortly before production ended. Most tracks were written by Tony Iommi and Bob Daisley (the vinyl version states that all songs were written by Iommi) although some lyrics were subsequently modified by Geoff Nicholls. Tony Martin said he "only sang on, and had no part in writing" The Eternal Idol, but nonetheless "thought [it] was one of the better albums of the band."
Drummer Bev Bevan and original Sabbath bassist Geezer Butler were hired to return for the 1987 tour in support of the album, effectively reinstating the Born Again tour line-up (with Martin in the vocalist spot rather than Ian Gillan). Butler backed out, however, upon learning that Sabbath had dates booked in South Africa, forcing Nicholls to bass for a few shows and eventually leading to the hiring of Jo Burt. Bevan would subsequently depart as well, replaced by former The Clash drummer Terry Chimes, who appears in the music video for "The Shining".
"I'd like to have seen some of the stuff off The Eternal Idol be a bit more credited," Iommi reflected to Sabbath fanzine Southern Cross, "because I think there's some good tracks on that album – 'Ancient Warrior'…"
There is a 3-minute and fifteen second studio outtake titled "Some Kind of Woman", which was written by Tony Martin shortly after joining the band. It was offered as a B-side to "The Shining" single and an early version of "Black Moon", which was released on Headless Cross, was released as a B-Side to the "Eternal Idol" single.
The album was re-released on 1 November 2010 in Europe in a 2-disc expanded set. Bonus content includes the aforementioned B-sides "Some Kind of Woman" and "Black Moon" on disc 1, while disc 2 contains the session for the album recorded with Ray Gillen on vocals.