Pope Francis on Sunday (Sept 5) officially declared Mother Teresa a saint of the Roman Catholic Church.

Over 100,000 pilgrims packed the Vatican to honour the nun who famously devoted her life to help the destitute in the slums of India's Calcutta.

But she will always be known as "Mother Teresa", he said.

"I think, perhaps, we may have some difficulty in calling her Saint Teresa: Her holiness is so near to us, so tender and so fruitful, that we continue to spontaneously call her Mother Teresa," the pope reflected.

Her canonisation came on the 19th anniversary of her death, marking a rapid ascent to sainthood hastened by her popularity and status as a Nobel Peace Prize laureate.

Here are six things to know about the road to becoming a saint.

1. There could be a five-year wait

Compared to Saint Teresa's 19-year wait, Saint Bede's 1,164 years seems like an eternity. Bede, a theologian, died in 735 and was canonised only in 1899.

There is usually a wait of five years after a person's death before the process begins. The delay allows for emotions to calm down after the death so that the case can be looked at objectively.

This was waived for Mother Teresa, and Pope John Paul II. John Paul II's cause for canonisation began a month after his death in 2005, and he was declared a saint in 2014.

For Mother Teresa, the process began less than two years after her death in 1997.

2. Servant of God to Venerable

After a person has been nominated for sainthood, he or she can be called a "servant of God".

Testimony and documentation about the life and virtues of the candidate has to be collected and examined, a process that takes years.

The Congregation for the Causes of the Saints reviews the evidence, and decides whether to proceed.

Once the "heroic virtues" of the individual is recognised by the Congregation and the Pope, "Venerable" is added to his or her title.

3. Beatification

The next step is beatification, and there has to be a miracle that occurred after prayers were made to the person after their death.

It is seen as proof that they are already in heaven, and hence able to intercede with God on behalf of others.

After beatification, the candidate is given the title "blessed".

4. Perform miracles after death

To be conferred sainthood, the candidate must have two officially approved miracles attributed to him or her after death.

Saints are defined by the Church as those who have led such holy lives that they have the ability to intercede with God in Heaven to perform miracles.

Mother Teresa's first miracle came less than a year after her death in 1997, and involved a tribal woman in West Bengal who had been diagnosed with stomach cancer.

The woman, Monica Besra, woke up one morning to find her tumour gone after she claimed to have felt a beam of light emanating from a photograph of Mother Teresa the day before.

Her status as saint was sealed in March this year when Pope Francis recognised a second miracle - that of a Brazilian man with multiple brain tumours, who was healed in 2008 after his wife placed a relic of Mother Teresa near his head and prayed to her regularly.

5. Who approves the miracles?

A Vatican-appointed Miracle Commission typically composed of theologians and scientific experts is in charge of verifying hundreds and thousands of miracle claims.

Miracles can only be confirmed if the healed person prayed solely to one person.

6. The "missing" miracle

But rules about conferring sainthood are not set in stone. Until 1984, two miracles were required for beatification and another two for sainthood.

And Pope Francis seems to think even two miracles are too onerous.

St John XXIII, called "the Good Pope", had one verified miracle, but Pope Francis waived the need for a second, and declared him a saint along with Pope John Paul II in 2014.

He has reportedly canonised other saints without authenticated miracles.

Instead, he prefers the process of "equivalent canonisation", which has three criteria: veneration by followers, reliable documention of their virtues, and a reputation for "wonders".

This approach is not unprecedented, and is in keeping with Pope Francis' reputation as a Pope of the poor, as the process for authenticating miracles can be expensive and protracted.