If someone believes they cannot know whether or not God exists, that person is an agnostic. Your grandmother might be a devout believer, while your mom is agnostic.
In 1869, English biologist T.H. Huxley coined this noun referring to someone whose religious beliefs lie somewhere in between those of a theist and an atheist — that is, a believer and a disbeliever. Huxley combined the Greek prefix a-, meaning "not," with gnostos, "known." This word can be used as a noun or adjective, and it can also refer to uncertainty about questions other than the existence of God: "Some philosophers remain agnostic as to whether people have free will."