Mother Teresa was a nun in a religious order in the Catholic Church. Nuns do not marry, they commit themselves to the single life and to a lifetime of service to God in the Church.
Mother Teresa originally joined the Sisters of Loreto, and took vows in 1937. The vows Mother Teresa took were vows of poverty, chastity and obedience. Poverty means that she would have no possessions of her own, but only possessions in community; chastity means choosing the single life and forfeiting the ordinary activities of married life. Obedience means obedience to the Word of God as revealed in the teachings of the Church and in the directions of superiors.
Mother Teresa felt a very strong call to serve Christ in the poorest of the poor. This is her charism, a special grace given to her by God. Mother Teresa used this grace to leave the convent and help the poor while living among them. It was an order. To fail would have been to break the faith." She began her missionary work with the poor in 1948, replacing her traditional Loreto habit with a simple white cotton sari decorated with a blue border, adopted Indian citizenship, and ventured out into the slums.