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 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,a where she grew up before becoming a nun and moving to India in her 20s.
It has taken 19 years since her death or 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 person cured of a tumour and another of suffering from hydrocephaly.
Mother Teresa was not without controversy. 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 judgment 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 Eastertide 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.