About 1238 he became lector at the Franciscan house at Oxford, and within a few years was regarded by the English province of that order as an intellectual and spiritual leader.
As Bishop of Lincoln, Grosseteste relied on his friend's opinion regarding ecclesiastical appointments in the diocese, since "Marsh knew everybody".
[5] Consulted as a spiritual director by Simon de Montfort, the countess of Leicester and the queen, as an expert lawyer and theologian by the primate, Boniface of Savoy, he did much to guide the policy both of the opposition and of the court party in all matters affecting the interests of the Church.
This collection shows the wide range of his correspondents, including Robert Grosseteste (whose own collection also includes several letters to Adam[7]), William of Nottingham (minister provincial of England), Simon de Montfort and his wife Eleanor, Bonaventure, and Henry III's wife Queen Eleanor.
His letters, written in highly stylized Latin according to the rules of the ars dictaminis, often served to advance his theological vision, particularly with regard to the pastoral work of the Church and his urgent sense of its need for correction and reform.