Oliver Cromwell is one of history's great dividers. Like Robespierre, Lenin, or indeed Thatcher, you either love him or hate him.

In the words of one contemporary, 'Never man was higher extolled, and never man was baselier reported of, and vilified than this man. No (mere) man was better and worse spoken of than he…'. Cromwell was to prove Charles's nemesis.

But, unlike Robespierre or Lenin, he was never a political or social revolutionary - it was religion which drove him to declare war on his King. Cromwell was a complicated, introverted and fanatical soldier for God. At times savage and brutal; at other times deeply compassionate to his troops and family. But from Marston Moor onwards, this brave, bad man was central to the English revolution.

Cromwell described himself as 'by birth a gentleman, living neither in any considerable height, nor yet in obscurity'. As the eldest son of the younger son of a knight, his fortunes were precarious and his status shifty. His father, Robert Cromwell, had been the MP for Huntingdon and his bequest to Oliver was not great.

In the early 1630s, Oliver was a struggling yeoman farming land around St Ives. But by the end of the 1630s, thanks to an inheritance from an uncle, he had become a prosperous county figure with substantial land around Ely and married to the daughter of a wealthy London fur trader. Crucially, Cromwell was also related to the Providence Island Company set of leading Puritans - he enjoyed connections with Pym, the Earl of Warwick, and the whole opposition set. When Cromwell first entered the House of Commons as MP for Huntingdon in 1628, he found nine kinsmen already there. From early days, Cromwell 'the outsider' was in fact a 'made man' on the Parliamentary side.

Oliver Cromwell's politics are more difficult to grasp. In 1631 he suffered financially and socially due to a miscalculation in local politics and had to move from Huntingdon to St Ives. There his social and political standing fell markedly. But as with most combatants of the Civil War, it was religion which drove his politics.

Sometime during the early 1630s, suspiciously at the time of his social and financial crisis, he experienced a spiritual rebirth accompanied by assurance of salvation. He later recounted his conversion:

'Oh, I lived in and loved darkness, and hated the light. I was a chief, the chief of sinners...Oh the riches of His mercy.'

Cromwell's abiding sense of the vital presence of God in his life drove him to believe in a divine plan for England in which he was convinced he would play a leading role. His personal salvation depended deeply on defeating a King who was corrupting England's Protestant heritage. And every action and every victory was a clear sign of God's support for his cause.

One of the hallmarks of Cromwell's politics was a reverence for the age of Elizabeth. To the Puritans, Elizabeth was as mythical as Margaret Thatcher is to an older generation of Conservatives - a ruler who could do no wrong and who governed a happy kingdom. In the time of Good Queen Bess, England's religion was settled, Crown and Parliament worked together, and across the globe, Protestant sea-dogs such as Raleigh and Drake crushed the Spanish menace. Cromwell's mother, his wife and his favourite daughter were all called Elizabeth. Charles's suspension of Parliament and Laud's religious innovations seemed a betrayal of this Protestant heritage.

In the 1630s, Cromwell began to make his name resisting the royal plan for draining fens as well as the Laudian policies of Matthew Wren, Bishop of Norwich. His opposition to the King and support of the Covenanters in the First Bishop's War earned him the title, 'Lord of the Fens.' His fierce temper and paranoid certainty helped turn him into a regional figurehead for discontent.

When Parliament resumed in 1640 he stood for Cambridge as a candidate known to oppose the King's will. Yet he was no republican. As a man of land, he always retained a deep and abiding belief in private property. But as the country slid into war and Cromwell saw the dangers of Popery everywhere, he began to place himself at the forefront of Parliamentary opposition to Charles. When war broke out, his powerful military leadership quickly became apparent.

Cromwell was not afraid of war. Cromwell energized his local militia and rose to command the cavalry in the Eastern Association under the Earl of Manchester. He showed what he was capable of at the 1643 Battle of Winceby where his disciplined cavalry charges routed the Cavalier forces to devastating effect. Soldiers were drawn to him, and Cromwell only chose the best. He quickly upset the delicate social niceties demanded by the aristocratic leaders of the Parliamentary army. When one criticised his low-born 'Ironsides', he famously replied, 'I had rather have a plain russet-coated captain that knows what he fights for and loves what he knows than what you call a gentleman and is nothing else'.

At Marston Moor, Cromwell showed he was a commander to be reckoned with. His stature could only grow.