In 1639, he published the most substantial history of Christianity in Britain to that date, "Britannicarum ecclesiarum antiquitates" – the antiquities of the British churches. It was an astonishing achievement in one respect – in gathering together so many previously unpublished manuscript sources. Ussher was very reluctant to arrive at firm judgements as to the sources' authenticity – hence his devotion of a whole chapter to the imaginative but invented stories of King Lucius and the creation of a Christian episcopate in Britain.