The album cover was intended to feature an Auguste Rodin sculpture from 1889 – also called "The Eternal Idol". Instead, it features naked models covered in paint and positioned to resemble the piece of art because permission to photograph the sculpture could not be secured. The paint had toxic effects and the models had to go to hospital at the end of the shoot.
The sleeve credits Dave Spitz as bass player, but all bass parts were by Bob Daisley. Eric Singer played the drums; the percussion credit for 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 initial sessions on Montserrat with producer Jeff Glixman. Gillen quit shortly after their return to England. He later joined the band Badlands.
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 modified by Geoff Nicholls. 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 Bevan and original Sabbath bassist Geezer Butler were hired for a 1987 tour in support of the album, which would have reunited three-quarters of the Born Again tour line-up (with Martin singing rather than Ian Gillan). However, Butler backed out on learning that Sabbath had booked dates in South Africa. Nicholls played bass for a few shows before Jo Burt was hired. Bevan was subsequently replaced by former Clash drummer Terry Chimes, who appears in the music video for "The Shining". ("The bass player in the 'Shining' video," recalled Martin, was some guy that we dragged off the street. I can't remember his name but he looked the part. He said that he was a guitarist. I remember he was always talking about how he was a Red Indian, thus all the turquoise he wore! We never saw him again.")
"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'…"
A three-minute and fifteen second studio outtake titled "Some Kind of Woman", written by Tony Martin shortly after joining the band, appeared as a b-side of the "The Shining" single. An early version of "Black Moon" – a song destined for Headless Cross – was released as a b-side of the "Eternal Idol" single.
The album was rereleased on 1 November 2010 in Europe as a two-disc expanded set. Bonus content includes the aforementioned b-sides "Some Kind of Woman" and "Black Moon" on disc 1. Disc 2 contains the session for the album recorded with Ray Gillen on vocals.