After taking the vows, Teresa left Darjeeling and reached Calcutta, where for the next 17 years she lived and worked at the Catholic College of Saint Mary's High School in the suburb of Entally, frequented mostly by the daughters of the British colonists. She taught history and geography, and was able to study the Hindi language.
The rule of the Sisters of Loreto did not allow it to get away from the convent but, thanks to the voluntary work done by some of his pupils had the opportunity to take more and more aware of the terrible living conditions in the slums of Calcutta, and in particular in the of Motijhil, bordering the school. In 1937 she went to Darjeeling to pronounce the vows. She thus became Mother Teresa, a name that remained for the rest of his life.
Back in Calcutta, gradually assumed different organizational responsibilities, to be appointed, in 1944, director of the school. The years of war had a profound effect on the activities of the nuns, who devoted themselves more and more to the reception of orphans and abandoned children. The same convent in Entally was requisitioned and, until 1945, became a British military hospital.
In 1950, Mother Teresa founded the Congregation of the Missionaries of Charity, whose mission was to take care of the poorest of the poor and all those people who feel unwanted, unloved, cared for by the company, all those people who have become a burden to society and who have fled from everyone. The first members were twelve girls, including some of his former students at Saint Mary. Established as uniform a simple sari white blue stripes, it seems he was chosen by Mother Teresa because it was the cheapest among those sold in a small shop but mostly because it had the colors of the untouchable caste, the poorest of India.
The number of people who wished to follow the example of Mother Teresa grew rapidly, so that the rooms initially made available by Gomes soon proved inadequate. In February 1953 the sisters could then move to a new office at 54A Lower Circular Road, made available to them by ' Archdiocese of Calcutta, which still houses the headquarters of the Missionaries of Charity. The lifestyle desired by Mother Teresa, inspired in part by St. Francis , provided strict austerity, in line with the condition of life of the poor and the need to preserve the ideals of the new order.