Tenacious D in The Pick of Destiny

As a young man, JB (Jack Black) leaves his religious family and oppressive Midwestern town for Hollywood on a quest to form the world's most awesome rock band. There he meets acoustic guitarist KG (Kyle Gass), who is performing on the street, and begins worshipping him as a rock god because of his skills and attitude. KG feeds JB's fantasy by pretending to be famous with a self-named band ("The Kyle Gass Project"), and exploits him to do work such as cleaning his apartment and buying him weed (under the promise that JB can audition for his fictitious band.) After JB learns KG is actually unemployed and living off his parents, the two become equal, and KG apologizes to JB by giving him a brand-new guitar. They create their own band: Tenacious D, named for birthmarks found on their behinds. (JB has a birthmark which says: "Tenac", and KG has a mark which reads: "ious D".)
Soon JB and KG learn the deepest secret of rock: all the rock legends used the same guitar pick, The Pick of Destiny, which has supernatural powers. It was created by a dark wizard from a piece of Satan's tooth, as a gift to a blacksmith who saved his life from the great demon. The pick gives its holder unnatural abilities with stringed instruments. Infatuated by the prospect of becoming the next great rock star, JB immediately sets Tenacious D on a quest to steal the Pick of Destiny from a rock history museum. Along the way, the band briefly splits up, when KG decides that sex comes first in "sex, drugs and rock 'n' roll", while JB wants to stick to the mission at hand.
But soon, JB and KG come back together when they realize they need each other as friends. The two manage to steal the Pick of Destiny due to incompetent security at the rock museum. Armed with this supernatural pick, they plan to use the winnings from a local bar's talent contest to pay their rent, but before they can go on stage, greed overcomes them, and they fight over the pick, snapping it in half accidentally. After a pep-talk from the bar's owner, they decide they are good enough to go onto the contest without the pick, not knowing that the owner is Satan in human form, looking for his missing tooth piece. Coming back outside (deciding they can both use half of the pick), they are confronted by Satan, who is "made whole" when he places the Pick of Destiny back on his broken tooth, giving him the ability to use his supernatural powers on Earth. He threatens to take Tenacious D back to Hell with him.
To save their lives, Tenacious D challenge Satan to a "rock-off", which he cannot deny, as the "demon code" strictly forbids him from declining such a challenge. As the bargain goes, if Tenacious D wins, Satan must return to hell on his own and pay their rent. But if Satan wins, he gets to take KG back with him. Ultimately, they lose (KG sings, "That was a masterpiece! / He rocks too hard because he's not a mortal man."), and Satan attempts to shoot KG with a bolt of energy. JB jumps in the way, and the bolt bounces off his guitar, blowing off a piece of Satan's horn. JB is able to send him back to Hell with a magical incantation. (As explained earlier, should Satan become "incomplete", he can be banished from the Earth with said incantation.)
The two turn Satan's horn into the "Bong of Destiny" and the movie closes with the two smoking from it as they write new songs.

Question: What deepest secret does JB and KG learn about?
Answer: Rock