In December 1634 Windebank was appointed to discuss with the papal agent Gregorio Panzani the possibility of a union between the Anglican and Roman Churches, and expressed the opinion that the Puritan opposition might be crippled by sending their leaders to the war in the Netherlands.
Among his correspondents were Galileo Galilei, Cardinal Francesco Barberini, nephew of Urban VIII, Famiano Strada, the historian of the Spanish wars in Flanders, Thomas Farnaby, the critic and grammarian, and Gregorio Panzani, who was sent by Urban VIII on a mission to the English Catholics.
He was also apparently at this time much mixed up in the tortuous negotiations with the papacy which were conducted through Gregorio Panzani; at the same time Montagu was asking license for his son to visit Rome, and the matter became in the hands of William Prynne a plausible accusation of romanising.
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