Mother Teresa was the founder of the Order of the Missionaries of Charity, a Roman Catholic congregation of women dedicated to helping the poor.
Mother Teresa taught in India for 17 years before she experienced her 1946 "call within a call" to devote herself to caring for the sick and poor. She established a hospice; centers for the blind, aged, and disabled; and a leper colony.
In May 1931, Mother Teresa made her First Profession of Vows. Afterward she was sent to Calcutta, where she was assigned to teach at Saint Mary's High School for Girls, a school run by the Loreto Sisters and dedicated to teaching girls from the city's poorest Bengali families. Mother Teresa learned to speak both Bengali and Hindi fluently as she taught geography and history and dedicated herself to alleviating the girls' poverty through education.

On May 24, 1937, she took her Final Profession of Vows to a life of poverty, chastity and obedience. As was the custom for Loreto nuns, she took on the title of "mother" upon making her final vows and thus became known as Mother Teresa. Mother Teresa continued to teach at Saint Mary's, and in 1944 she became the school's principal.

However, on September 10, 1946, Mother Teresa experienced a second calling that would forever transform her life. She was riding a train from Calcutta to the Himalayan foothills for a retreat when Christ spoke to her and told her to abandon teaching to work in the slums of Calcutta aiding the city's poorest and sickest people. "I want Indian Nuns, Missionaries of Charity, who would be my fire of love amongst the poor, the sick, the dying and the little children," she heard Christ say to her on the train that day. "You are I know the most incapable person—weak and sinful but just because you are that—I want to use You for My glory. Wilt thou refuse?"

Since Mother Teresa had taken a vow of obedience, she could not leave her convent without official permission. After nearly a year and a half of lobbying, in January 1948 she finally received approval from the local Archbishop Ferdinand Périer to pursue this new calling. That August, wearing the blue and white sari that she would always wear in public for the rest of her life, she left the Loreto convent and wandered out into the city. After six months of basic medical training, she voyaged for the first time into Calcutta's slums with no more specific goal than to aid "the unwanted, the unloved, the uncared for.

Mother Teresa quickly translated this somewhat vague calling into concrete actions to help the city's poor. She began an open-air school and established a home for the dying destitute in a dilapidated building she convinced the city government to donate to her cause. In October 1950, she won canonical recognition for a new congregation, the Missionaries of Charity, which she founded with only 12 members—most of them former teachers or pupils from St. Mary's School.

As the ranks of her congregation swelled and donations poured in from around India and across the globe, the scope of Mother Teresa's charitable activities expanded exponentially. Over the course of the 1950s and 1960s, she established a leper colony, an orphanage, a nursing home, a family clinic and a string of mobile health clinics.
In February 1965, Pope Paul VI bestowed the Decree of Praise upon the Missionaries of Charity, which prompted Mother Teresa to begin expanding internationally. By the time of her death in 1997, the Missionaries of Charity numbered over 4,000—in addition to thousands more lay volunteers—with 610 foundations in 123 countries on all seven continents.

The Decree of Praise was just the beginning, as Mother Teresa received various honors for her tireless and effective charity. She was awarded the Jewel of India, the highest honor bestowed on Indian civilians, as well as the now-defunct Soviet Union's Gold Medal of the Soviet Peace Committee. And in 1979, Mother Teresa won her highest honor when she was awarded the Nobel Peace Prize in recognition of her work "in bringing help to suffering humanity.
For her unwavering commitment to aiding those most in need, Mother Teresa stands out as one of the greatest humanitarians of the 20th century. She combined profound empathy and a fervent commitment to her cause with incredible organizational and managment skills.
Summing up her life in characteristically self-effacing fashion, Mother Teresa said, "By blood, I am Albanian. By citizenship, an Indian. By faith, I am a Catholic nun. As to my calling, I belong to the world. As to my heart, I belong entirely to the Heart of Jesus."