Can't get the idea of this mod off my mind...

[caravel]

You can use Judaism and Heretical Cults for a religion, as I am in DATW (I'm the script writer). The problem is both lack cultures, which can be a problem. You can work around this by setting them up with any other culture, you just have to remember what you assigned them. For example, the Steppe factions in Dark Age are heretics (Steppe Culture Type) with a pagan culture, and they work fine. They properly spread their own religion, through time, agents and buildings, but EVERYONE hates them, thus they at least appear to be on the move. Just a note, Judaism is not as hated.

When using said religions coupled with a different culture, remember how this can effect unit recruitment, and also remember to add the lines "Place Culture Here"_CULTURE after the end of every unit you give them in the Startpos file. It works for me, but ask Macsen, he did with his Israelites in Ancient Total War, which you may be able to get a few ideas from.

That's good to know. So do your heretic Steppe factions have their heretic piety showing on their generals' info parchments in the correct form?

And this brings me to the next part. If religion were to be used as "culture" then how would piety still be viable? Perhaps piety could be seen as how much of a true Roman your general is as well as loyalty to the people/faction? Currently a pious general is the best choice of governor for a pious province. This could work as for example a "true Greek" being a better governor to "true Greeks" but a poor governor to non Greeks? Yes I think that would work well.

On the subject of jihads it's a doable idea but you might have a problem with them being spammed if they're too easily available. Also they'd be rather ahistorical as they were in MTW anyway. I'd also keep the Muslim relgion and culture as the Cathaginian/Eastern/Egyptian culture and not use it for the barbarians. They would be better as the Pagans. I'm not sure but I think you can probably mod Jihads to be used by other religions anyway, as they're not linked to a papacy like entity in the same way crusades are.