He was a son of William Sergeant, a yeoman in Barrow-upon-Humber, Lincolnshire, and was admitted in 1639 as a sub-sizar at St John's College, Cambridge, graduating in 1643.
[1] On the recommendation of William Beale he was appointed secretary to Thomas Morton, the Anglican Bishop of Durham, time he spent on transcriptions of the Church Fathers.
For the next twenty years he was actively engaged in controversy, both with Anglicans such as the bishops Edward Stillingfleet and John Tillotson, and Catholics who differed from Thomas White.
At the time of the Oates Plot he entered into communication with the Privy Council which greatly scandalized the Catholics.
And shewing the perfect Conformity of Christian Faith to Right Reason, and the Unreason ableness of Atheists, Deists, Anti-trinitarians, and other Sectaries.