So how exactly do you create or join a secret religious cult?
If there is no cult yet you'll need to be a vassal and your liege will have to have a different religion. You then have a decision available to falsely convert. You can then take another decision that creates the secret cult.
I'm not sure what exactly changes if the secret cult already exists, but one option is to simply wait for a member of the socieety to try and convert you. That's a bit slow though and you could also try falsely converting and checking out the societies tab.
Been a long time since I played. If you're playing with equality for women (or heaven forfend enatic via some mod, last I heard), does the AI still totally suck at handling it?
I once played a Warrior Empress with numerous Warrior Queens underneath her that I doled out, setting up every roadblock to masculine control I could - female-preferred succession, etc. Almost all of them married patrilineally and within two generations they were 70% male and most of them with agnatic-cognatic.
Which really took the wind out of my sails. Not to mention having to just grab women out of the ether via present debutante (not having the option to get them via more specialized pulls) and hope they weren't totally awful for running a country.
With the improvements to the character finder it's now easy to find people willing to join your court though women still face the restriction that they need to be unmarried.
However, I expect the AI is still bad with preserving female rule because it simply wasn't designed for that. The game in general presumes male preference in many of it's systems.
If you want actual equality (or female preference) you'll have to mod the game further. For example add a decision that allows a liege to demand vassals adopt whatever gender law the liege is using, much like lieges can now demand vassals switch to gavelkind succession.
You'll also want to either eliminate the prestige penalty for marrying under your station or set the AI not to care about prestige when arranging marriages. That way female rules will matrilineally marry lowborn men instead of waiting for a chance to matrilineally marry a noble's son of suitable rank, then suddenly realize their biological clock is ticking and get into a regular marriage.
Another thing that could be modded in is to restrict available gender laws for vassals based on the gender law of the top liege. Perhaps make it so vassals can be no more than 1 step away from their direct liege's gender law and no more than 2 steps away from their top liege's gender law.
Finally you could mod the order in which gender laws are listed. Whenever you hand out a duchy or county to someone the new noble will adopt the default gender law, which is the one at the top of the list. And in the unmodded game that is Agnatic-Cognatic. You could switch the order and put True Cognatic at the top, which would make it the new default.