She was dubbed the "saint of the gutters" after working tirelessly to care for the destitute in the slums of Calcutta.
But Mother Teresa became a saint in earnest yesterday after she was canonised by Pope Francis at a ceremony which drew pilgrims to the Vatican in their tens of thousands.
The Albanian-born nun had been fast-tracked through the complex, expensive canonisation process, which saw her elevated to sainthood just 19 years after her death.
Speaking in St Peter's Square to an audience of more than 120,000 Catholics - as millions more watched on television - the Pope declared her a saint for "the poorest of the poor."
Saint Teresa spent her life "bowing down before those who were spent, left to die on the side of the road, seeing in them their God-given dignity," he told the crowds in and around the Via della Conciliazione.
"She made her voice heard before the powers of the world, so that they might recognise their guilt for the crimes of poverty they themselves created," he added. As if to emphasise the point, the Pope repeated the phrase "the crimes of poverty they themselves created".
He delivered the speech while standing under a canvas which showed the late nun wearing her distinctive, blue-hemmed white robes.
A huge security operation surrounded proceedings, with more than 3,000 police officers on patrol in the Vatican and the streets of Rome.
Speaking after the ceremony, Sister Leandra Stupnicka, from Wroclaw in Poland, said she got up at 4am to pay her respects to the new saint.
"This mother was simple, very poor and devout and for us she is a testament to serving others," Sister Leandra told Reuters. "We pray to be like her."
"Everything she did gave an example to the entire world," 17-year-old student Massimiliano D'Aniello said, "little gestures made with so much love are what's important."
In an act of compassion, the Pontiff arranged for a pizza lunch to be served to 1,500 needy people after the Mass.
They came from shelters run by Mother Teresa's order in Rome, Milan, Bologna, Naples and Florence. The food was cooked in three mobile ovens by 20 chefs from a Naples pizzeria.
Saint Teresa's ceremony has proven to be the highlight of the Vatican's Year of Mercy, a special calendar of events decreed by the Pope Francis, who last week described indifference to suffering as a "modern sin."
However, a shadow lies over parts of her legacy, with critics accusing her of prolonging the suffering of her patients in the slums of India by withholding painkillers. Indian nationalists accused Saint Teresa of trying to convert vulnerable Hindus in her charge to Christianity.
Christopher Hitchens, the late British journalist and author, infamously described her as "a fanatic, a fundamentalist and a fraud" in his book The Missionary Position. In a 2013 study, Canadian academics criticised Mother Teresa's "rather dubious way of caring for the sick, her questionable political contacts, her suspicious management of the enormous sums of money she received, and her overly dogmatic views regarding, in particular, abortion, contraception, and divorce".
The Vatican has shrugged off the criticisms, insisting that she embodied the highest standards of Christian self-sacrifice and devotion.
John Paul II, who met frequently with Mother Teresa, never doubted her eligibility for sainthood and broke with tradition by putting her on the route to canonisation two years after her death, instead of five.