Against the intellectual backdrop of mounting uncertainty concerning the secular nature of modernity, the present study reconstructs a philosophical debate in Germany, concerning the origins and legitimacy of the modern age as a secular age. This debate was carried out between Hans Blumenberg, Karl Löwith and Carl Schmitt, between about 1950 and 1980. Blumenberg’s little-known early work is considered as a preliminary to his best-known book, Die Legitimität der Neuzeit, in which he attacked Schmitt and Löwith for propounding versions of a theorem whereby modern thought consisted of “secularized” religious matter. It is also shown to develop other core concepts, notably that of human self-assertion, whilst being charted against Blumenberg’s own intellectual self-assertion, his struggle to find a distinctive voice. A historical discussion of the concept of secularization is followed by an examination of Löwith’s derivation of the progressivist philosophy of history from Christian eschatology, which is shown to be less straightforward than Löwith’s popularizers assumed. Löwith’s personal confrontation with Blumenberg having been shown to conceal a deeper affinity in the rejection of apocalyptic thought, the focus shifts to Schmitt’s political-theological critique of modernity. Schmitt deploys the secularization theorem to call into question the human autonomy on which Blumenberg founds the modern age’s claim to legitimacy. Schmitt is shown to argue against what he perceived to be a human plan to terminate history in an earthly utopia, precluding a divine judgment on history and man. The subsequent debate between Blumenberg and Schmitt in letters and texts reveals fundamental differences concerning the status of man with regard to the absolute, with Schmitt’s concept of antagonistic, absolute potencies being pitched against Blumenberg’s vision of a division of powers, which he discusses in terms of the myth of Prometheus. In arguing for the autonomy of the modern age, Blumenberg is shown to challenge critiques of civilization which avail themselves of religious rhetoric for political ends.