[6] He was a nephew, by marriage, and associate of William Fiennes, 1st Viscount Saye and Sele, and his early works are close to the direct political concerns of this patron.
[6][9] At the same time he was secretary to Robert Devereux, 3rd Earl of Essex, who emerged in 1642 in a prominent position as Parliamentary military leader.
It was published, 39 letters being made public, edited and annotated by Parker, Sadler and Thomas May, as The Kings Cabinet Opened.
[17] In 1649, he with Henry Robinson argued for 'engagement', a kind of loyalty oath to be imposed by the victorious Parliamentary side to ensure recognition of its authority.
[19] His Discourse concerning Puritans drew on Virgilio Malvezzi and Paolo Sarpi, and attacked both episcopacy and Presbyterianism as autonomous systems.
[21] In The Case of shipmony (1640) he argued in terms of salus populi, the law of necessity, and the failure of the King's arguments to establish it.
Two important examples were works from Dudley Digges and John Spelman, in A view of a printed book intituled Observations upon His Majesties late answers and expresses (1642).
He argued strongly against the concept that the king had his power through popular consent, and placed Parker in a tradition going back to William of Ockham and Marsilius of Padua.
[32][33] Christopher Hill states that Philip Hunton's Treatise of Monarchie (1643) is an attempted compromise between Parker and John Goodwin.