The story of how the political difficulties of the crown were exponentially increased in the later 1630s and the early 1640s by Catholicism and Catholic activism has been told a number of times, and notably by Caroline Hibbard. Professor Hibbard has shown how the papal agent George Con, almost from the first moment that the Scots rebelled, manipulated the breakdown of proposals for an Anglo-French alliance. He contrasted loyal Catholic subjects with rebellious (Scottish) Protestants, and suggested that papal authority might be called upon to prevent foreign intervention in Scotland. Con's activities were a prime source for the formulation of contemporary ‘popish-plot’ conspiracy theories, as was the involvement of Catholics (predominantly Irish and Scottish, but also some English and Welsh) in the attempt to put down the Covenanters. When Scottish Catholic peers were heavily involved in grossly unsuccessful but high-profile attempts in Scotland to take on the Covenanters, and when English crypto-Catholic peers such as the earl of Arundel were prominent among those leading the king's forces up country to confront the Scottish rebels, neither the rank and file nor even the officer corps had to be predominantly professing papists to excite the kind of widespread popular hostility which manifested itself in the riots and disturbances charted so well by Robin Clifton.

There has, however, been a debate about the extent of Catholic involvement in the war. It was axiomatic among pro-parliament propagandists that royalist forces were riddled with Catholics, even dominated by them.