The life and work of the German chemist and philosopher Wilhelm Ostwald (1853–1932) is studied from the angle of scientism. In Ostwald's case scientism amounted to: (i) the construction of a unified science of nature (energetics); (ii) its use as the ‘scientific’ basis for an all-embracing philosophy or world-view (energeticism); (iii) the programme to realize this philosophy in practice, as a secular religion to replace Christianity. Energetics, a generalized thermodynamics, was proposed by Ostwald and others to replace mechanics as the fundamental theory in physical science. Its contents and fortunes are briefly discussed. Ostwald's energeticist philosophy is illustrated by his classification of the pure sciences, his theory of happiness and his science-based ethics. Ostwald cherished a metaphysics in which energy and energy transformations were fundamental to all phenomena and processes. As a result, he transferred his optimism about science and technology to energeticism: philosophical, psychological and ethical questions were regarded as capable of solution by his scientistic philosophy. The practice of Ostwald's energeticism is illustrated by two of his ‘Sunday sermons’ (on prayer and on Christmas), as well as by a public lecture on science. He considered science the modern and legitimate substitute for God and claimed that science was, or would soon be, omnipresent, eternal, almighty, omniscient and perfectly good. Ostwald's substitute religion of science suggests the question, discussed in the last section of this paper, to what extent and in what sense historians might fruitfully interpret scientism as a secular religion.