From a radically Protestant family of Cornwall, England, though of part Dutch origin, Peter emigrated to a Puritan colony in America, where he first rose to prominence.

Peter unsuccessfully proposed revolutionary changes that would have disestablished the Church of England's role in landholding and struck at the heart of the legal title to property.

Disagreeing with the war against Protestant Holland and increasingly excluded after Cromwell's death, Peter's former outspokenness and role in the execution of Charles I meant he faced reprisal following the Restoration and he was hanged, drawn and quartered as a regicide.

[1] In September 1643 the Parliamentary Committee of Safety employed Peter on a mission to Holland, there to borrow money on behalf of Parliament, and to explain the justice of its cause to the Dutch.

[7] He also became famous as an exhorter at the executions of state criminals, attended Richard Challoner on the scaffold, and improved the opportunity when Sir John Hotham was beheaded.

Whenever a town was to be assaulted, it was his business to preach a preparatory sermon to the storming parties, and at Bridgwater, Bristol, and Dartmouth his eloquence was credited with inspiring the soldiers.

During the siege of Bristol he made converts of five thousand clubmen, and when Fairfax's army entered Cornwall, his despatches specially mentioned the usefulness of Peter in persuading his countrymen to submission.

He performed useful services in the Second Civil War, procured guns for the besiegers at the siege of Pembroke, raised troops in the Midlands, and arranged the surrender of the Duke of Hamilton at Uttoxeter.

The same year, during the Third Civil War he was in South Wales, endeavouring to bring over the people to the cause, and subsequently was present at the Battle of Worcester in 1651, where afterwards he preached to the victorious Parliamentary soldiers.

[14] Peter rode at the head of the force bringing Charles I to London as prisoner, and justified and supported the trial and sentence in sermons.

[17] The country became unstable and factional after Cromwell's son fell from power, and General Monck came from Scotland leading the only effective and unified force left.

Peter's preaching and addresses to Parliament on Cromwell's behalf had made him too well known as a Puritan opponent of the royal house of Stuart for any disavowals to save him; thus, a recantation of his opinions and a display of repentance would probably have been his best hope.

In addition to justifying Charles being condemned to death, Peter was alleged to be one of the two heavily disguised executioners, even the one who welded the axe, though this was a task requiring some skill.

A witness testified that Peter had ordered a carpenter to drive staples into the scaffold (for tying Charles to the block if he resisted), been present at the scene on the day of the execution, disappeared for an hour during it, and was seen drinking water with the presumed headsman, Richard Brandon, afterwards.

Peter produced an alibi, claiming that he had been ill and confined to bed at home on that day, which was confirmed by a house servant of his, but the court found that testimony unconvincing.

He is said to have been a man of a rough, coarse nature, without tact or refinement, of strong animal spirits, undeterred by difficulties which beset men of higher mental capacity, whose energies often outran his discretion, intent upon the realities of life and the practical side of religion.

In the opinion of that writer, Peter's conception of religious controversy, that all differences could be avoided if ministers could only pray together and live together, is highly characteristic, and shows the largeness of his personal sympathies and at the same time the limits of his intellectual imagination.

Burnet characterises him as "an enthusiastical buffoon preacher, though a very vicious man, who had been of great use to Cromwell, and had been very outrageous in pressing the king's death with the cruelty and rudeness of an inquisitor",[23] His jocularity had given as much offence as his violence, and pamphlets were compiled which related his sayings and attributed to him a number of time-honoured witticisms and practical jokes.

[24] His reputation was further assailed in songs and satires charging him with embezzlement, drunkenness, adultery, and other crimes; but these accusations were among the ordinary controversial weapons of the period, and deserve no credit.

[26] An examination of the career and the writings of Peter shows him to have been an honest, upright, and genial man, whose defects of taste and judgement explain much of the odium which he incurred, but in the opinion of Firth do not justify it.

[1] In about 1625, while Peter was preaching in Essex, he married Elizabeth, widow of Edmund Read of Wickford, and daughter of Thomas Cooke of Pebmarsh in the same county.