He became prominent as an ecclesiastical lawyer in the service of Archbishop John Whitgift, active against the Puritans in the Church of England.
[5] In the major confrontation of the 1590s between Anglicans and Thomas Cartwright and his Puritan and presbyterian allies, Cosin with Matthew Sutcliffe for the church lawyers faced the common lawyers Richard Beale and James Morice.
Presbyteriall Discipline exploited the scare after the 1591 plot of William Hacket, Edmund Coppinger, and Henry Arthington.
[8] Cosin noted in it that the presbyterian notion of discipline included the ideas of resistance to bad magistrates, and deposition of kings.
He expressed the views that Magna Carta implied that the English monarchy did not have absolute power, but that it had no application to ecclesiastical jurisdiction.