'Saint of the gutters', Mother Teresa canonized 19 years after her death
Pope urges others to follow her lead and not to turn away from the poor
To the hard-bitten, journalist Malcolm Muggeridge, she was a revelation, an example of how faith can be transformative. To an idealistic young convent schoolgirl like me, watching his documentary in the early 1970s, she was the embodiment of Christianity, caring for the dying, unwanted children, abandoned lepers.
When Muggeridge's film and subsequent book about Mother Teresa of Calcutta first appeared more than 40 years ago, the world sat up and took notice.
Here was a remarkable story of a nun who had given up everything to found her own missionary order in India and serve the poorest of the poor.
From then on, in a world that usually bestowed its admiration on the rich and those with artistic and sporting talent, she became one of the most celebrated people on the planet. She was known as a living saint or the 'saint of the gutters' and received that secular accolade of sanctity, the Nobel Peace Prize, in 1979.
Today her sanctity is confirmed when Mother Teresa is canonized by Pope Francis and becomes Saint Teresa of Kolkata in front of a congregation at St Peter's in Rome that will include members of her Missionaries of Charity order and representatives of the Indian government and of the Balkan nations of Albania and Macedoni - where she grew up before becoming a nun by joining the Loreto Sisters in Rathfarnham, Dublin and moving to India to join the Loreto order there in her 20s.
Her links with Ireland remained and in 1993, she was given the Freedom of Dublin during a ceremony on Dawson Street, and she also met President, Mary Robinson.
Pope Francis last night, ahead of the ceremony, told the sea of cheering supporters gathered beneath a portrait of Mother Teresa, her head graced by a halo: "She deserves it!"
"We'll have the joy of seeing Mother Teresa proclaimed a saint," he said.
He took the opportunity to urge others to follow her lead, decrying those who "turn the other way not to see the many forms of poverty that begs out for mercy". Choosing "to not see hunger, disease, exploited persons, this is a grave sin. It's also a modern sin, a sin of today", he said.
Among those in the square for the prayer vigil, were nuns from Teresa's Missionaries of Charity order as well as volunteers who helped rescue survivors of the recent deadly earthquake in central Italy, whom the Pope hailed as "artisans of mercy".
It has taken 19 years since her death for her to be canonized - not the fastest to be raised to the altars, but far from the slowest either. Pope John Paul II became a saint nine years after his death in 2005; Joan of Arc, who died in 1431, was not canonized until 1920.
The Catholic Church investigates those put forward for sainthood by collecting evidence, arguing the case with theologians, talking to doctors about miracles claimed.
Two are required for canonization and in Mother Teresa's case, these include a woman cured of an abdominal tumour through Mother Teresa's intercession in 1998, on the first anniversary of the nun's death. Pope Francis also decreed last year that she would be given a sainthood, after attributing to her the "miraculous" cure of a Brazilian man who was suffering from a viral brain infection that had left him in a coma.
Mother Teresa was not without controversy, however. She was criticised, particularly by Christopher Hitchens, the newspaper columnist, for failing to address the causes of poverty and dealing only with its symptoms.
She was too willing to take tainted money from the likes of Robert Maxwell and did not spend it on the most up-to-date medical care for the dying, he argued.
But as far as the Catholic Church is concerned, the mud didn't stick.
The Vatican's Congregation for Causes of Saints, which oversees canonization, says that she matches the definition of a saint: an example of holiness, a channel of God's love, through whom we can learn what God is like.
But why has the Church decided that now is the time for Mother Teresa to join around 10,000 people who have been named saints?
As well as their holiness, saints can be useful to the Church in conveying particular messages.
During her lifetime, Teresa was particularly approved of by Pope John Paul II. She was fiercely opposed to abortion and contraception, as was John Paul, who declared they were part of the culture of death. In embracing every unwanted child, every dying person she encountered, Mother Teresa for him embodied the culture of life and John Paul oversaw her beatification - the final stage before full sainthood - in 2003.
She is also a useful exemplar for Pope Francis.
He has made mercy the theme of his pontificate and declared December 8, 2015 to November 20 this year, to be the Year of Mercy.
He has called upon people to be moved "from indifference to compassion".
The Church, for many, has historically represented judgement and a focus on sin, but Pope Francis urges it to offer "more evident signs of God's presence and closeness". These, he said during Easter 2015, should be especially offered to the suffering, the alone and abandoned and those "without hope of being pardoned or feeling the Father's love".
Mother Teresa is therefore an ideal saint for what the Pope is trying to teach during his Year of Mercy.
There is another reason, though, she is seen by the Church as a saint for this particular age.
While she was perceived as someone of immense faith, the nun suffered from a terrible burden.
She believed herself to be unloved, feared God not wanting her.
This spiritual trial, what the Church calls a dark night of the soul, went on for years. It came to light after her death, in letters saved by her spiritual director.
The woman who focused on the unloved and unwanted shared those feelings. She not only felt compassion for those who felt lonely and abandoned, she identified with them. Her compassion and her heroic spiritual struggle make her a saint for modern times.
While no major events are planned in Kolkata, reflecting a heated debate over religious intolerance in Hindu-majority India, prayers will be held by the city's small Christian community, marking today's ceremony.
Catherine Pepinster is editor of 'The Tablet', the Catholic weekly. Additional reporting by the 'Sunday Telegraph'