a person who believes that people only do things to help themselves, rather than for good or sincere reasons (Oxford)
2. a person who believes that people are only interested in themselves and are not sincere (Cambridge)
Cynics will say that there is not the slightest chance of success. (Oxford)
I'm too much of a cynic to believe that he'll keep his promise. (Cambridge)
cynics will say that there is not the slightest chance of success
a cynic to believe that someone will keep his promise
Pessimist
Skeptic
Believer
Optimist
Cynical