I have read numerous novels featuring nefarious cult religious groups, mercenary groups, and hidden pasts. This one is very powerful, and I evidently started with Book 4 in the series. ( Yeah, I'll be looking for the first three!)
Lydia Strong is a true crime author, and her husband's a private investigator. They end up involved with a religious cult, stolen diamonds, faked suicides, weapons dealers, and psychopaths. Lydia has an ex-student that had called for help, and then suddenly disappears. LOTS of twists and turns in plot and assumptions!