thanatism

from The Century Dictionary.

• noun The view that death means not only an arrest of all physiological functions, but also the definite disappearance, cessation, or destruction of the human mind or soul. Thanatism is opposed by Haeckel to athanatism, the belief in man' s personal immortality. Primary thanatism is the original absence of the dogma of immortality in certain primitive uncivilized races; secondary thanatism is the later outcome of a rational knowledge of nature in the civilized intelligence.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The belief that the human soul eventually dies.