They supported the Republic in the expectation that it was a preparation for the "fifth monarchy" - that is, the monarchy which should succeed the Assyrian, the Persian, the Grecian, and the Roman, and during which Christ should reign with his saints on earth for a thousand years.
The "fifth monarchy" is a biblical reference to a prophetic dream of Nebuchadnezzar in Daniel 2:44. The Fifth Monarchists saw the overthrow of Charles I as a divine sign of the second coming of Christ and the establishment of a thousand years of Christ's rule on earth.
Being disappointed at the delay in the fulfilment of their hopes, they attempted to foment political agitation against the government of Cromwell. They found considerable support amongst members of the New Model Army and The Levellers.
One of their leaders was Major-General Thomas Harrison, the son of a Newcastle-under-Lyme butcher. He was involved in the execution of Charles I, commanding the guards on the day of the royal execution in 1649. Other prominent members were Christopher Feake, Vavasor Powell, John Carew and John Rogers.
The arrest of Feake and Powell, two of the most violent of their number, was sufficient for a time to damp their ardour. They had supporters in the Barebones Parliament (July-December 1653) but their political power peaked in 1653 with the Nominated Assembly, where many of the delegates were from congregations with Fifth Monarchy sympathies. They were horrified at the establishment of Cromwell's Protectorate and plotted to overthrown the regime. Two plots were uncovered and broken up in 1657 and 1659.
After the Restoration, on January 6, 1661, fifty of them, headed by a wine-cooper named Thomas Venner, made a mad effort to attain possession of London in name of "King Jesus". Most of the fifty were either killed or taken prisoner, and on January 19 and 21 Venner and ten others were executed for high treason.
The failure of Venner's Rising led to repressive legislation to suppress non-conformist sects. The doctrine of the sect then either died out, or became merged in a milder form of Millenarianism.