Ambrosians are members of one of the religious brotherhoods which at various times since the 14th century have sprung up in and around Milan and also a 16th-century sect of Anabaptist Ambrosians.
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Only the oldest of the Catholic Ambrosians, the Fratres S. Ambrosii ad Nemus, had anything more than a very local significance. This order is known from a bull of Pope Gregory XI addressed to the monks of the church of St Ambrose outside Milan.
Saint Ambrose, Bishop of Milan, certainly did not found religious orders, though he took an interest in the monastic life and watched over its beginnings in his diocese, providing for the needs of a monastery outside the walls of Milan, as Saint Augustine recounts in his Confessions. Ambrose also made successful efforts to improve the moral life of women in the Milan of his time by promoting the permanent institution of Virgins, as also of widows. His exhortations and other interventions have survived in various writings: De virginibus, De viduis, De virginitate, De institutione virginis, De exhortatione virginitatis, and De lapsu virginis consecratae. Ambrose was the only Father of the Church to leave behind so many writings on the subject and his attentions naturally enough led to the formation of communities which later became formal monasteries of women.
It is against this background that two religious orders or congregations—one of men and one of women, when founded in the Milan area during the 13th and 15th centuries—took Saint Ambrose as their patron and hence adopted his name.
The first of these groups was formed in a wood outside Milan by three noble Milanese, Alexander Grivelli, Antonio Petrasancta, and Albert Besuzzi, who were joined by others, including some priests. In 1375 Pope Gregory XI gave them the Rule of St Augustine, with set of constitutions. As a canonically recognized order they took the name "Fratres Sancti Ambrosii ad Nemus" and adopted a habit consisting of a brown tunic, scapular, and hood. The brethren elected a superior with the title of prior who was then instituted by the Archbishop of Milan. The priests of the congregation undertook preaching and other tasks of the ministry but were not allowed to accept charge parishes. In the liturgy they followed the Ambrosian Rite. Various monasteries were founded on these lines, but without any formal bond between them. In 1441 Pope Eugene IV merged them into one congregation called "Congregatio Sancti Ambrosii ad Nemus", made the original house the main seat, and laid down a system of government whereby a general chapter met every three years, elected the priors who stayed in office till the next chapter. There was a rector, or superior general, who was assisted by two "visitors".