In 1891, Tétreault joined the Sisters of Bethany and stayed there for 10 years. It was during her time serving in a poor neighborhood of Montreal that she realized her dream of an apostolic school for women and a seminary for the foreign missions. Around this time, she met Father Gustave Bourassa who was her guide and led her to all the important persons she needed to fulfill her dreams. In 1902, Paul Bruchési, the Archbishop of Montreal gave permission for the founding of the congregation. Two years later, he went to Rome and spoke to Pope Pius X about this new foundation. The pope immediately answered, "Found, found, and all the blessings of Heaven will fall upon this new Institute and you will call them the Missionary Sisters of the Immaculate Conception."