Ever since the purchase of the Manor of Ingatestone in 1539, the Petre family has played a prominent part not only in the history of the Catholic community in Essex, but in the life of the Church further afield. Sir William Petre, the founder of the Essex line, and two of his descendants have merited the attention of a biographer, while there has also been a substantial periodical literature associated with the family. However, no such detailed study has yet been written on perhaps the most intriguing member of the family, the little-known thirteenth Baron of Writtle, Monsignor Lord William Joseph Petre. The present article seeks to shed light on the early part of the career of this pioneer of Catholic liberal education and the first Catholic priest to take his seat in the House of Lords since the Reformation. The focal point of Petre's earlier years was the monastery and school of St. Gregory's, Downside.