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’. Published in The British Journal for the History of Science. 46(2): 231–254, June 2013. Click here or on title.