Help me remember the name of the book… please

I read this book a long time ago… but it was at a very troubled time in my life, I went through some traumas and my mind erased most of the memories I have from that time. But I wanted to read this book again, because it left me feeling very, very good.

What I remember is that it's a police investigation, a thriller, and the protagonist is a woman. I remember there was something like a secret sect or a secret order and there was a priest (or something like that) who helped her a few times. I know that in the end he was the villain and she saw him in a square, something like that.

Please help me…