You can read the character bio's if you want to know.
The reasonings of the cult's membership are explained in the Codex of Molag Bal which was made by Theo I think.
Since the Deadra are the Princes of Change, one of the prime goals of the cult is to bring about change. They want to "change" the current authorities and bring about a New World Order, one in which Molag Bal is supreme.
I agree that those Sermons are difficult to follow, but I think you have the right idea about what the book should be like. Basically the same kind of metaphorical ramblings which would only make sense to the ingame author (Heceril) or people that know a lot about Lore.
Edit:
If anyone plans on making the book, please add a lot of contradiction with the two Sermons.
Finished:
Kushtashpi
The first shrine that I visited during my travels. Kushtashpi lies far north and along the shores of the Grazelands, where the dome gazes over the sea with majestic grace. The shrine is modest and lies at the end of a long winding tunnel. Here I offered an emerald to our Lord.
Yansirramus
Travel far west from Tel Aruhn, across the treacherous sea, and you shall find Yansirramus on the last island before reaching the shore. With the calm waters surrounding it, it almost feels as if the sea itself fears the might of our Lord. The shrine itself is of modest sized. Here I offered Lord Bal the only possession I had at the time, a silver blade.
Esutanamus
Swim north from Yansirramus until you reach the land. Then travel north-eastward across the lands until you reach the shore again. Swim to the large Island that dominates the landscape, and you will find Esutanamus, Overseer of the Grazelands. Within the shrine there are two halls, travel through the first hall to reach the statue of our Lord. I offered Lord Bal a pearl I had taken from the sea. Only our Lord is deservant of such beauty.
Tusenend
There are two routes to reach Tusenend, the haven of our Lord's offspring, which lies in the Molag Amur. I chose to show my loyalty through taking the long route. One must depart from the fortress of the weak, Molag Mar. Travel north-eastward and let Lord Bal guide you through the middle passageway. From hereon, continue forward, do not fear our Lord's offspring which rapes these lands, and turn to the right at the final end of the route to find Tusenend. The shrine consists of a single main hall, in which our Lord's dominance provides a welcome haven. I offered Lord Bal a diamond taken from an Ordinator's corpse.
Bal Ur
Bal Ur, ancient fortress of Lord Bal, which lies north of the Hlaalu settlement of Suran. Cross the mountains to gaze upon his citadel of the southern Molag Amur. Enter the fortress and you shall find a large two story complex, though no altar to our Lord. To find his statue, you must find the door to the underground. The underground tunnels are long and perilous, but within time you shall find Lord Bal's majestic throne. Underneath the statue of our Lord, I placed my most coveted piece of protection, a glass helmet. One need not have mortal armor when one is protected by something far greater, the blessing of our Lord.
Ramimilk
Travel to the Redoran city of Ald'ruhn, then travel directly south. You shall find a fortress of our Imperial foes. Travel south-westward from thereon, and you shall find Ramimilk, Lord Bal's jewel of the Ashlands. Enter the shrine and you shall swiftly gaze upon our Lord's statue. At the alter of Lord Bal I sacrificed an Imperial with a glass blade, his screams of agony became rythms of beauty in the hall of our Lord.
Ashalmawia
The final shrine I visited on my travels, our Lord's manor of the West Gash, Ashalmawia. Ashalmawia lies to the east of the Redoran settlement of Ald Velothi. Upon entry one shall quickly be greeted by Lord Bal and should find his manor a welcoming haven. Go through the left doorway, travel down the stairs, and you shall discover the sunken vaults of Molag Bal's manor. At the shrine I surrendered a steel claymore to our Lord's guidance, and thereby concluded my travels on Vvardenfell.