In 1891, the New York positivist T. B. Wakeman chose, as W. M. Brown would later do, to embrace 'scientism' (the doctrine and the label). Wakeman wrote:
We find the religious history of our race to consist, therefore, of a gradual evolution of its leading peoples from a broad base of general animism and fetichism [sic], thence to astrology, thence to polytheism, thence to monotheism, and thence to scientism expressed chiefly to us in the pantheism of Goethe, the positivism of Comte, the synthetism of Spencer, the cosmism of Fiske, and finally by the monism of Haeckel. He proposed this word monism as expressive of the world-unifying law of science, as the summary of all that was true and good in the other philosophic names proposed by the philosophers just named, while it excluded what he regards as the crude and vulgar notions of materialism, spiritualism, and dualism. (Thaddeus B. Wakeman, 'Ernst Haeckel', in Evolution in Science, Philosophy, and Art: Popular Lectures and Discussions before the Brooklyn Ethical Association [NY: D. Appleton & Company, 1891], p. 41; italics in the original)Uses from the 1930s are interesting, since they precede the papers by Hayek that popularized 'scientism'. In his 1935 doctoral dissertation, Contribution of the Ideologues to French Revolutionary Thought, Charles van Duzer had this to say:
Two major currents ran through French thought during the latter half of the eighteenth century. On the one hand was Rousseauistic sentimentalism which tended toward the close of the century to pass into a religious mysticism as exemplified in the writings of Bernardin de Saint-Pierre and Saint-Martin; on the other, the Voltairian philosophy of common sense and scientism which informed the rational and utilitarian tendencies of Encyclopedic thought. (van Duzer, Contribution of the Ideologues to French Revolutionary Thought in The Johns Hopkins University Studies in Historical and Political Science Series 53, No. 4 [Baltimore: The Johns Hopkins Press, 1935], p .5 of the number [p. 419 of the volume])Next, there are two uses from the late 1940s. First, we have Max Horkheimer's use in 1947: 'Like any existing creed, science can be used to serve the most diabolical social forces, and scientism is no less narrow-minded than militant religion' (Horkheimer, Eclipse of Reason, 1947).
In the same year, we find Werner Jaeger using the term as follows: 'The unilateral emphasis on the physical side of pre-Socratic philosophy in their works is a product of 19th-century scientism and its horror of everything metaphysical' (Jaeger, The Theology of the Early Greek Philosophers (Oxford University Press, 1947), p. 195, n. 25).
In a letter to William L. Kinter (dated July 30, 1954), C. S. Lewis wrote that That Hideous Strength 'is about a triple conflict: Grace against Nature and Nature against Anti-Nature (modern industrialism, scientism, & totalitarian politics)' (C. S. Lewis: Collected Letters, v. III: Narnia, Cambridge, and Joy 1950-1963, ed. Walter Hooper [New York: HarperCollins, 2006], pp. 497-98).
By the 60s, uses of 'scientism' had become common , but it's interesting to see how Leszek Kolakowski used the word (in 1968):
...scientism, that is, the doctrine according to which any question that cannot be settled by the methods of the natural and deductive sciences is an improper question' [p. 160]. ... It [logical empiricism] professes scientism, that is, it asserts the essential unity of the scientific method, accounting for differences between the sciences on this score -- especially between the social and the physical sciences -- by the immaturity of the former [pp. 177-8] (Kolakowski, The Alienation of Reason, ).