The term 'doubting Thomases' is often used to describe individuals who are skeptical or require proof before believing in something. However, there are several other synonyms that can be used to describe such individuals. One of the most common synonyms is 'skeptic' - a person who questions or doubts accepted opinions. Another word that can be used to describe doubting Thomases is 'cynic' - someone who has a negative or distrustful outlook on things. Alternatively, the term 'pessimist' can also be used to describe individuals who tend to have a negative view of things and doubt positive outcomes. Ultimately, doubting Thomases, skeptics, cynics, and pessimists all share a similar trait - they require evidence or proof before they believe something to be true.