Modern historians have long recognised that conceptions of the ‘ancient’ history of both parliament and the Protestant Church were vital to the political, legal and religious argument of the period, but the relationship between these two types of historical thinking has rarely been established. This article contends that the need to establish a pre-Reformation history of the Royal Supremacy, so as to counter Catholic challenges of religious innovation, required Elizabethans to create related myths of kings-in-parliament through the ages, exercising jurisdiction over the national Church. It was therefore under Elizabeth that the antiquity of parliament, its centrality to an ‘ancient constitution’, was first asserted by Elizabethan divines to validate the parliamentary framework of the English Protestant Church. It is argued that historical argument about parliament’s origins and evolution derived from the polemical battles fought by various religious interest groups on both sides of the confessional divide who defended, criticised or denounced the type of Church established in 1559. The history of parliament, then, first emerged in the war of ideas waged around the Royal Supremacy.