This essay examines Secularism as developed by George Jacob Holyoake in 1851–1852. While historians have noted the importance of evolutionary thought for freethinking radicals from the 1840s, and others have traced the popularization of agnosticism and Darwinian evolution by later Victorian freethinkers, insufficient attention has been paid to mid-century Secularism as constitutive of the cultural and intellectual environment necessary for the promotion and relative success of scientific naturalism. I argue that Secularism was a significant source for the emerging new creed of scientific naturalism in the mid-nineteenth century. Not only did early Secularism help clear the way by fighting battles with the state and religious interlocutors, but it also served as a source for what Huxley, almost twenty years later, termed ‘agnosticism’. Holyoake modified freethought in the early 1850s, as he forged connections with middle-class literary radicals and budding scientific naturalists, some of whom met in a ‘Confidential Combination’ of freethinkers. Secularism became the new creed for this coterie. Later, Secularism promoted and received reciprocal support from the most prominent group of scientific naturalists, as Holyoake used Bradlaugh's atheism and neo-Malthusianism as a foil, and maintained relations with Huxley, Spencer and Tyndall through the end of the century. In Holyoake's Secularism we find the beginnings of the mutation of radical infidelity into the respectability necessary for the acceptance of scientific naturalism, and also the distancing of later forms of infidelity incompatible with it. Holyoake's Secularism represents an important early stage of scientific naturalism.