The backstory isnt really complicated at all. Danny's is a little more so. Johnnys is easy.
Crash was dying so he conjured Satan. Roxanne knew about it and made a side bet with Zadkiel. Johnny sold his soul, but before Satan could claim it, Zad bonded his soul with a Spirit of Vengeance. Satan cannot take something that is bonded with a heavenly spirit. Satan wants what is rightfully his, and the only way for him to get it, is for johnny to die. So for years he has been trying to kill Johnny, and feeding him lies about what the Ghost Rider really is. Namely, that hes bonded to a powerful demon named Zarathos. IMPORTANT: Johnny was never bonded to a demon named Zarathos, Zarathos was a lie. But it is possible Zarathos exists. Maybe S.O.V. have temporary amnesia after bonded with a host, that would explain why Johnny and Danny's S.O.Vs never told them the truth, or by heavenly law their not allowed to tell. I believe the S.O.V. have amnesia and the Caretakers are around to guide and protect the host and the Ghost Rider. This would also explain why Noble's spirit wanted possesion of hell, it was convinced it was a demon. A powerful demon at that. Even issue 99, maybe 100 of Danny's run, his mothers spirit tell Danny that the S.O.V. is an angel.
Any Questions? I would be happy to clear up any misconseptions. Dan Way and Jason Aaron's run on Ghost Rider explains all of this when read with Simon Spurriers