On June 3, 1902, in a modest apartment of Côte-des-Neiges, Delia Tétreault and her first two companions, Ida Lafricain and Joséphine Montmarquet, inaugurated the first Canadian Missionary Institute, an Apostolic School, aiming to prepare young women for Religious missionary life. The following year, the group already numbered eight persons. So, on May 3, 1903, the Apostolic School moved to 27 Côte St. Catherine Road, Outremont, in a bit bigger house. Upon the request of neighbouring families, a small school was opened for the children of the area. The dining room was used as a classroom for the eight younger pupils and the boudoir, for the six older ones.

On March 17, 1904, Miss Ida Lafricain left for Saint Boniface (Manitoba), on the insistent request of the Archbishop, Most Reverend Adelard Langevin, O.M.I. He needed her services for the house and chapel as well as the Teacher Training College he wanted to open. A few years later, Miss Ida Lafricain became co-foundress and first Superior General of the Oblates of the Sacred Heart and of Mary Immaculate, a Religious Congregation that spread through certain regions of Western Canada.

With the founding of the Apostolic School, Delia Tétreault had cleared the way for her work but she felt she had to go further. In silence and prayer, in rereading events, she searched for signs of God's project so as to adapt herself to the situation without delay. Father Gustave Bourassa backed Delia in the foundation of the Apostolic School. He had become her friend, father and indispensable support. In October 1904, in order to check on the repairs being done on the organ in his church, he went up into the choir loft. On his way back down, he missed a step and fell all the way down. He died on November 20, as a result of this accident. His Grace Paul Bruchési, archbishop of Montreal, was in Rome for the celebration of the fiftieth anniversary of the dogma of the Immaculate Conception. Upon hearing of the death of Father Bourassa, he told himself : The poor children have lost their father, I shall be their protector.

In an interview with Pope Pius X, Archbishop Bruchési spoke of the nascent community but without manifesting much enthusiasm : If you tell me to dissolve what has begun, it will be done quickly; if you tell me to continue, I will do so… Pius X answered without hesitation : Found, found, Your Grace, and all the blessings of heaven will descend upon this new foundation. This happened on November 30th. On the following December 7th, during a second audience granted to Archbishop Bruchési, the Holy Father while speaking about this foundation, said : You will name it the Society of the Missionary Sisters of the Immaculate Conception.

On August 8, 1905, which was the anniversary of his Episcopal consecration, Archbishop Bruchési presided the Eucharistic Celebration during which Delia Tétreault took her perpetual vows and became Mother Mary of the Holy Spirit. Joséphine Montmarquet (Sr. St. Gustave) took temporary vows while Blanche Clément (Sr. St. Paul), Zenaïde Marcoux (Sr. Marie-de-Lourdes) and Émilda Charbonneau (Sr. St. Joseph) made their promises as novices.

In the springtime of 1906, the little community bought the house next-door because of a lack of space due to several new recruits on the horizon. The two houses were later joined together by a wooden passageway. The domestic premises soon became insufficient. A big house, across the street, was bought in 1906. It was the beginning of the actual Motherhouse at 314 Côte Saint Catherine Road. The Foundress, a prudent and shrewd woman, also wanted to guarantee the social status of the young Congregation. On February 28, 1907, the legislative Assembly of the Province of Quebec officially approved its incorporation under the name : Les Soeurs Missionnaires de l'Immaculée-Conception. The foundation became stronger and rooted in the Church and in civil society. The stem became evermore vigorous. At this point, the call to open up to the whole world came to the Congregation from Rome through Archbishop Bruchési : All mission countries are open to you. You are told now as once the apostles were : Go, teach all nations.