Thomas White alias Blacklo, an English Catholic priest, natural philosopher and theologian, was the leader of a small group of Catholics, known as ‘Blackloists’, who in the 1640s and 1650s wrote in support of Oliver Cromwell. This article seeks to explain the ecclesiological, theological and political arguments put forward by White and his followers in order to justify their approach to the Independents and later to the Lord Protector. After putting into context and interpreting some of the issues elaborated in White's circle, the reaction of the Holy See to Thomas White and to his political and theological positions is examined. While contemporary historiography seems to agree that the majority of English Catholics were on the king's side during the civil war, the evidence that emerges from White's case shows that the Roman hierarchy was of a different opinion.