The article takes issue with current orthodoxy concerning early modern republicanism, centred on Quentin Skinner's model of classical republicanism. I argue that historians of political thought need to return to first principles in their practice in order to understand early modern republicanism, and I provide an example by using those principles to reassess one canonical text, Philippe de Plessis Mornay's Vindiciae, contra tyrannos. Reading the Vindiciae in context reveals it as a work whose radicalism lies, not in its engagement with the Roman law tradition, but in its express conviction that each and every individual is responsible for maintaining a covenanted relationship with God. My reassessment tracks the political, and specifically regicidal, consequences of commitment to that belief in England from the late sixteenth through the mid-seventeenth centuries. It destabilizes the anachronistic distinction between ‘political’ and ‘religious’ modes of thought that historians of political thought too often use to characterize early modern political discourse, and it points to the common ground shared and articulated by theorists including, inter alia, John Ponet, George Buchanan, and John Milton. The conclusion considers what this investigation reveals about republicanism as a political phenomenon in Europe and America from the sixteenth through the eighteenth centuries.