Your court chaplain will find the secret Shia, albeit at a pretty slow rate - unless he is also a secret Shia, in which case he probably won't find any.
You can't act against them directly (outside of court chaplain actions), but you can plot-murder them. Or you could exploit the AI to revoke their titles - see below.
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Preparation:
- Get 1-2 thousand prestige.
- Pass the religious revocation law.
- Be at peace.
- Be stable enough to be able to survive 1-2 months of being of the "wrong religion".
- Have enough personal power (retinues, mercs, levies) to be able to put down a few vassal revolts.
- Have a few thousand spare cash for pre-conversion gifts.
Revocation:
- Mark of the secret Shia as "characters of interest".
- Once you become publicly Shiite, you will not be able to see the secret Shiites.
- Personally adopt Shia.
- Demand conversion from all of the secret Shia
- Most of them will probably agree, but you may need to offer gifts to some of them (eg. zealous).
- Don't stay Shia for too long, or your Catholic vassals will start factionating...
- Convert back to Catholic (eg. from capital - requires 500-1000 prestige).
- Optional: If you want to maintain your membership of the assassins: pause, become Catholic, become secret Shia, unpause.
- Revoke all of your Shia vassals' titles.
- Beware of revolts from powerful vassals / vassals in powerful factions.
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EDIT:
I just noticed that you're in the secret Shia society, not the assassins. This opens up a different option: become the head of the secret Shia (eg. by targeted murder) and then use the rank 4 power to force the entire society into the open, convert back to Catholic, and revoke. Don't forget to flag secret Shia who aren't members of the society - you can use the demand conversion approach described above during your 1-2 weeks as Shia.
If you are independent and powerful enough, the current head of the secret Shia might force everyone into the open at the earliest opportunity (ie. as soon as they have enough society currency). In this case, there's no need to plot-murder your way to the top.
I think it's also worth pointing out that all of the revokation approaches described above are not at all what your current character would do. If you want to role-play, it would be much more fun to transform your realm from Catholic to openly Shia. Yes, even if you are Scandinavia.