skeptic
1: an adherent or advocate of skepticism
2: a person disposed to skepticism especially regarding religion or religious principles
skepticism
1: an attitude of doubt or a disposition to incredulity either in general or toward a particular object
2a: the doctrine that true knowledge or knowledge in a particular area is uncertain
2b : the method of suspended judgment, systematic doubt, or criticism characteristic of skeptics
3: doubt concerning basic religious principles (such as immortality, providence, and revelation)
Courtesy of Merriam-Webster