He was born at Norwich, and was educated at Norwich grammar school and at Caius College, Cambridge, where he was scholar and afterwards fellow. On taking orders he was appointed secretary to Bishop Overall of Lichfield, and then domestic chaplain to Richard Neile, Bishop of Durham. In December 1624 he was made a prebendary of Durham, and in the following year archdeacon of the East Riding of Yorkshire.
In 1628 he took his degree of D.D. He first became known as an author in 1627, when he published his Collection of Private Devotions, a manual stated to have been prepared by command of King Charles I, for the use of Queen Henrietta Maria's maids of honour. This book, together with his insistence on points of ritual in his cathedral church and his friendship with William Laud, exposed Cosin to the hostility of the Puritans; and the book was criticised by William Prynne and Henry Burton. In 1628 Cosin took part in the prosecution of a brother prebendary, Peter Smart, for a sermon against high church practices; and the prebendary was deprived.
In 1634 Cosin was appointed master of Peterhouse, Cambridge; and in 1640 he became vice-chancellor of the university. In October of this year he was promoted to the deanery of Peterborough. A few days before his installation the Long Parliament had met; and among the complainants who hastened to appeal to it for redress was the ex-prebendary, Smart. His petition against the new dean was considered; and early in 1641 Cosin was sequestered from his benefices. Articles of impeachment were, two months later, presented against him, but he was dismissed on bail. For sending the university plate to the king, he was deprived of the mastership of Peterhouse (1642). He went to France, preached at Paris, and served as chaplain to some members of the household of the exiled royal family. At the Restoration he returned to England, was reinstated in the mastership, restored to all his benefices, and in a few months raised to the see of Durham (December 1660). At the convocation in 1661 he played a prominent part in the revision of the prayer-book, and endeavoured with some success to bring both prayers and rubrics into better agreement with ancient liturgies. He administered his diocese successfully for eleven years; and used a large share of his revenues to promote the interests of the Church, of schools and of charitable institutions. He died in London.
Though a ritualist and a rigorous enforcer of outward conformity, Cosin was uncompromisingly hostile to Roman Catholicism, and most of his writings illustrate this antagonism. In France he was on friendly terms with Huguenots, justifying himself on the ground that their non- episcopal ordination had not been of their own seeking, and at the Savoy conference in 1661 he tried hard to effect a reconciliation with the Presbyterians. He differed from the majority of his colleagues in his strict attitude towards Sunday observance and in favouring, in the case of adultery, both divorce and the re-marriage of the innocent party.
Among his writings (most of which were published posthumously) are a Historic Transubstantiationis Papalis (1675), Notes and Collections on the Book of Common Prayer (1710) and A Scholastical History of the Canon of Holy Scripture (1657). A collected edition of his works, forming 5 vols of the Oxford Library of Anglo-Catholic Theology, was published between 1843 and 1855; and his Correspondence (2 vols) was edited by Canon Ornsby for the Surtees Society (1868-1870).
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