The symbols came about because the band (Jimmy Page, Robert Plant, John Paul Jones and John Bonham) decided their 1971 album couldn't be called "Led Zeppelin IV." Jimmy Page explains on "Led Zeppelin: The Black Mountain Side" (plaza.v-wave.com/zeppelin/d-iv.html): "We decided that on the fourth album, we would deliberately play down the group name, and there wouldn't be any information whatsoever on the outer jacket. Names, titles and things like that do not mean a thing. . . . I had to talk like hell to get that one done."
Adds Robert Plant: "Each of us decided to go away and choose a metaphysical type of symbol which somehow represented each of us individually--be it a state of mind, an opinion, or something we felt strongly about, or whatever. Then we were to come back together and present our symbols."
John Paul Jones' symbol (circle over three interlocking ovals) was found in a book of runes and purportedly represents a person who is both confident and competent. It also mysteriously appears on the cover of a book about the Rosicrucians.
Bonham's symbol (three interlocking circles) came from the same book, and Bonham just liked it (though Plant says the circles symbolize the man-woman-child trilogy). At a stop in Pittsburgh, the band noticed it was also the emblem of Ballantine beer. You be the judge.
Plant's symbol (circle around a feather) features the feather of Ma'at, the Egyptian goddess of justice and fairness. It is also the emblem of a writer. Plant says the symbol he created was drawn from sacred symbols of the ancient Mu civilization, a fantastical place that supposedly existed until 14,000 years ago. The last remnants are said to be the primitive statues at Easter Island. Those who believe in Mu also believe in Lemuria, Atlantis and maybe even the Easter (Island) Bunny.
Page designed his own symbol (Zoso). Though it resembles the alchemical symbol for mercury, its meaning remains a mystery. The most recent fandom theory is that it symbolizes a near-death or Tantric sex experience to unify the worlds of the living and the dead, and thus to reveal the secrets of the universe.
In the words of the immortal Keanu Reeves: Whoa.
Page's only public comment on his symbol came during a November 1994 appearance on "Denton" with Plant. At the end of their interview a member of the studio audience yelled "What's your symbol mean, Jimmy?" After some confusion as to what was being yelled, understanding dawned on Page's face and he replied simply "Frying tonight."
Which makes sense if the secrets of the universe have to do with burnt crunchy bits.
"You may not believe this," says Plant, "but Pagey once took me aside and said 'Look, I'm going to tell you the meaning of this once, and then I shan't ever mention it again--or at least, not for a long, long time anyway.' And would you believe that I have since forgotten what it was, and now Pagey won't tell me."
Page has, however, spoken out on your question of his doing the Faustian thing. He collects occult books and paraphernalia. He was a devotee of Aleister Crowley (a famous English occultist and member of the Hermetic Order of the Golden Dawn) to the point of buying Crowley's old house. But, Page says he is not a Satanist.
Zeppelin biographer Charles Cross has this to say about such rumor-mongering: "From mud sharks to pacts with Satan, the public seems to suspend normal human empathy when talking about Led Zeppelin, accepting preposterous malice of the sort that would never be accepted if it were directed toward anyone outside of rock 'n' roll. Have you heard anyone describe a successful business executive as having sold his soul to Satan?"