While at Cambridge he embraced puritan principles, and became a communicant with the congregation of Jonathan Jephcott at Swaffham Prior.
Holcroft eventually formed a church on congregational principles, and, after being ejected in 1662 from Bassingbourn, became a bitter opponent of episcopalianism.
[2] In 1663 Holcroft was imprisoned in Cambridge gaol, by order of Thomas Chicheley, for illegal preaching, but he was occasionally allowed by the warder to visit his congregations.
At the assizes he was sentenced to abjure the realm, but on Arthur Annesley, 1st Earl of Anglesey representing his case to Charles II he was allowed to remain in gaol.
By means of a writ of certiorari he was removed as an insolvent debtor to the Fleet Prison in London, and frequently preached there to large crowds of people.
He left a small estate to the poor of his congregations, and a piece of ground at Oakington for a burial-place.