A doubting Thomas - It is used to refer any person who is a little bit of paranoid and never believes anyone or anything unless and until a suitable proof is supplied to him/her.
1. Please Martyn, why are you behaving like a doubting Thomas on me?
2. He is really a big doubting Thomas, I told him that she was coming to her house, but he kept me asking about the phone call.
3. Don't behave like a doubting Thomas in front of me, I don't like it.