Cromwell was a Puritan. Puritans were Protestants who wanted to purify the Church of England of Roman Catholic practices. They believed that the Church of England was too similar to the Roman Catholic Church, and that the reformation was not complete until it became more protestant.

Oliver Cromwell was a staunch and radical Protestant, specifically a Puritan. He was born into the Church of England but underwent a profound religious conversion in the 1630s, becoming a "Congregationalist" who believed that every individual could have a direct relationship with God without the need for a complex church hierarchy. Cromwell’s Protestantism was characterized by an intense hatred for Catholicism, which he viewed as "popery" and a threat to English liberty. This religious fervor fueled his brutal military campaigns in Ireland, where he sought to dismantle Catholic influence by seizing land and suppressing Catholic worship. As Lord Protector, his government enforced "godly" living, famously banning Christmas and other festivals he considered to have pagan or "papist" origins. His legacy remains deeply polarizing, particularly in Ireland, where he is remembered as a symbol of Protestant oppression against the Catholic population.