Mother Teresa of Calcutta, one of the most iconic figures of the 20th century, is set to become a Catholic saint on September 4.

The open-air Mass led by Pope Francis is expected to attract hundreds of thousands of the faithful.

Pope Francis will lead the open-air Mass. Photo: Getty

Born Agnes Gonxha Bojaxhiu in 1910, in Skopje, Macedonia, the ethnic Albanian Teresa helped the poor in India for most of her life.

She founded the Missionaries of Charity in 1950 and gained worldwide recognition for her work, including a Nobel Peace Prize in 1979.

“The canonisation of Mother Teresa invites us to look to her as a Christian hero, an outstanding model of the Christian life,” the Canadian priest who promoted her sainthood cause, Father Brian Kolodiejchuk, said in March.

Teresa was a revered figure throughout the world, but not universally liked.

Her hardline opposition to abortion and contraception, as well as readiness to accept donations from dictators, have been a matter of controversy.

Private letters published after her death in 1997 also revealed that for the last 50 years of her life she despaired over having lost a personal connection with Jesus, while she continued to serve his cause.

Her canonisation is one of the highlights of Francis’ Jubilee of Mercy, a Catholic festival running until November 20. Kolodiejchuk said the timing was “fitting,” given the pope’s own focus on the destitute.

In a preface to a book on the soon-to-be saint, published in July, Francis recalled how giving to the needy is a key Christian teaching.

“Mother Teresa made this page of the Gospel the guide for her life and the path to her holiness – and it can be for us, as well.”

Francis will lead a Mass in St Peter’s Square during which he will be asked three times, in Latin, to add Teresa to the pantheon of Catholic saints.

WITH AAP