Pope John XXIII and Pope John Paul II may have been widely admired and respected around the world, but what makes them worthy of the mantle of sainthood that they will be receiving this Sunday? There are four stages in the canonisation process.

Step 1: Servant of God

First, a potential sainthood candidate must be declared a 'Servant of God'. This begins when the bishop of the diocese where the candidate died begins an investigation, which is usually prompted by a petition from the faithful.

This investigation usually doesn't start until at least five years after the potential saint has died, but the Pope does have the power to waive that rule, as Pope John Paul II did with Mother Teresa, and Pope Benedict XVI did with John Paul II.

To carry out the investigation, there is usually a guild or foundation, or other organisation set up. Evidence is gathered in the forms of writings, sermons, and eye witness testimony. From all this a detailed biography is drawn up.

Then, when enough information is gathered, the biography is taken by the presiding bishop to be submitted to the Roman Curia, the administrators of the Catholic Church. At this point, the candidate is known as a 'Servant of God'.

Within the Roman Curia, there is the Congregation for the Causes of the Saints, a committee that deals with all matters of sainthood. They assign a Postulator, a specific person to oversee this candidate's case for sainthood.

The Postulator gathers more information about the candidate, and then later permission is given to have the body of the prospective saint exhumed and examined. Once there is confirmation that no improper cults or societies have formed around this person, relics are taken and the second stage is moved onto.

Step 2: Venerable/Heroic in Virtue

At this step, Congregation for the Causes of the Saints recommends to the Pope that they should make a declaration describing the candidate as 'Heroic in Virtue'.

This means that they were an exceptional Catholic, and to do that they have to display a 'heroic' amount of two groups of attributes known as the cardinal virtues and the theological virtues.

The cardinal virtues are prudence, justice, fortitude and temperance, and the theological virtues are faith, hope and charity.

If the sitting Pope is satisfied that these virtues are displayed 'heroically' in the candidate's life, they will make a declaration, and the candidate will be given the title 'Venerable'.

This is the stage where the Catholic faithful are encouraged to start praying in the name of the candidate, with things like prayer cards being distributed. However, at this stage there is no special feast day for the candidate.

Step 3: Beatification/Blessed

This is the stage where the candidate will receive the title the 'blessed' and they can be honoured by a particular group of people, or a particular place.

If someone is blessed, it is a statement that the Church believes that it is "worthy of belief" that they are in fact in heaven.

To be beatified, candidates must meet one of two criteria. Either they have to be responsible for a miracle, or they have to have been martyred.

The most common form of accepted miracles are usually medical in nature, but there are very strict rules on what constitutes a bona fide miracle.

There has to have been prayers to the candidate as a venerable, and then the cure or healing has to have been spontaneous, instantaneous, total and permanent, and something that cannot be medically explained.

To be confirmed, a given miracle is examined by a group of Italian doctors called the Consulta Medica, not all of whom are Catholic. While they don't pronounce a miracle, they do make the official statement: "We can find no scientific or medical explanation for the cure."

Other miracles that are sometimes considered include incorruptibility, where the candidate's body has not undergone decomposition. Catholics believe this happened in the case of St Catherine of Siena. Despite having died in 1380, her body remained pristine and without decomposition more than 600 years later, without embalming of any kind.

Another body related miracle is known as the odour of sanctity, where a candidate's body emits a sweet scent, rather than the stench of decay. This supposedly happened to St Teresa of Avila for nine months after her death in 1582.

There is also the miracle of liquefaction, where the dried blood of a candidate on a relic or stored in a vial miraculously turns back into liquid. St Januarius of Naples who died in 305 is the traditional example. His dried blood is said to liquefy every year on the 19^th of September.

If the miracle or martyrdom is confirmed to be legitimate, then the Pope will declare the candidate to have been 'beatified'.

This now leaves only one last step.

Step 4: Sainthood

For the Pope to accept someone to be canonised as a saint, it has to be proven that they are responsible for at least two miracles.

In the case of John XXIII, often otherwise known as 'Good Pope John', Pope Francis has provoked controversy by approving his canonisation with only one miracle to his name.

That miracle was the healing of Italian nun Sister Caterina Capitani of a stomach tumour in May of 1966, who prayed to the deceased pontiff for healing three years after he had passed away.

Sister Adele Labianca, who had been taking care of Sister Capitani, said to the Religious News Service: "Not only was it a physical healing but an internal healing when you could feel the presence of God."

For John Paul II, there are two confirmed miracles on record. The first was the healing of French nun Sister Marie Simon-Pierre from Parkinson's disease in 2005.

Speaking on the BBC, Sister Marie Simon-Pierre described the moment she knew she was healed: "Exactly two months has passed since John Paul II returned to the House of the Father. I awoke at 4:30, amazed at having been able to sleep. I suddenly got out of bed: my body was no longer painful, there was no rigidity and, inside, I was no longer the same."

Then in 2011, the second miracle was the healing of a Costa Rican mother of four, Floribeth Mora Diaz, who had an aneurism in a delicate and inaccessible area of the brain.

The RNS quoted her telling her story: "My greatest concern was not dying but concern about what would happen to my children."

After praying to the deceased pontiff one night, Ms Diaz reported seeing a vision.

"When I woke up in the morning, I looked at the magazine cover which showed Pope [John Paul II] with his arms outstretched.

"I felt a deep sense of healing. I heard his voice say to me, 'Get up and don't be afraid.'

"I went to my husband in the kitchen and told him I was cured. I realised little by little the illness had been taken away."

Once both miracles are verified as genuine by the Consulta Medica, the Pope will then make a declaration that the candidate now enjoys what the Catholic Church calls 'beatific vision', the ability for the most perfect and direct communication between an individual and God.