How can I take my passport back from my employer...

We would like to inform you that if you are an expat and the employer took your passport, then the employer doesn't have the right to take it. So you can claim for it, and if they refuse then you can file a complaint with the police and provide a copy of that complaint to the Ministry of Labor to prove the violation.
Note that the law allows the governmental departments that belong to the Ministry of Interior to take the passport, otherwise it is legal violation such as the private sector.

You must follow up the complaint before the Labor Court to obtain the passport by a decision from this authority, no more than that