Haeckel’s scientific religion

Haeckel, however, was not simply a biologist in the sense that we would use that word today. For he saw himself—and was seen by many German intellectuals and artists in the latter part of the nineteenth century and the early part of the twentieth century—as the founder of a new scientific religion. He called his philosophy monism and saw himself as the leader of a movement of aggressive rationalism which would eventually rid Germany of the last traces of superstitious religion and replace Christianity with a religion which glorified modern science.

Richard Webster
Why Freud was Wrong:
Sin, Science and Psychoanalysis
Basic Books, p. 229–230, 1996.