Mother Teresa formally made a saint by Pope Francis

Pope Francis is urging people to resist 'the temptation to indifference' to suffering in a speech on the eve of a ceremony to formally proclaim Mother Teresa a saint.

Mother Teresa will be made a saint. Photograph: AFP/Getty

Greeting thousands of lay volunteers from around the world in St Peter’s Square yesterday, he said ignoring those suffering hunger, sickness or exploitation is a “modern sin, a sin of today”.

The square hosts a ceremony today expected to draw hundreds of thousands of admirers of Mother Teresa, the nun who cared for the destitute on the streets of India. She died in 1997.

Among yesterday’s crowd were firefighters and volunteers who rescued survivors from the 24 August earthquake in central Italy.

When Pope Francis canonises Mother Teresa today, he will be honouring a nun who won admirers around the world and a Nobel Peace Prize for her dedication to the “poorest of the poor”.

He will also be recognising holiness in a woman who felt so abandoned by God that she was unable to pray and was convinced, despite her ever-present smile, that she was experiencing the “tortures of hell”.

For nearly 50 years, Mother Teresa endured what the church calls a “dark night of the soul” – a period of spiritual doubt, despair and loneliness that many of the great mystics experienced, her namesake St Therese of Lisieux included. No-one but Mother Teresa’s spiritual directors and bishop knew of her spiritual agony until her correspondence came to light during her beatification cause. The letters appeared in a 2007 book, Come Be My Light.

Rev Brian Kolodiejchuk, the Canadian priest who published the letters, said that in canonising her, Pope Francis is recognising Mother Teresa not only shared the material poverty of the poor but the spiritual poverty of those who feel “unloved, unwanted, uncared for”.

“That was her experience in her relationship with Jesus,” he said. “She understood very well when people would share their horror stories, their pain and suffering of being unloved, lonely. She would be able to share that empathy because she herself was experiencing it.”

Pope Francis has made the canonisation the high point of his Jubilee of Mercy, a year-long emphasis on the church’s merciful side.