The canonisation Sunday of Archbishop Oscar Romero, assassinated in 1980, and of Paul VI, who was pope from 1963 to 1978, is the last stage in the Vatican's arduous process of creating saints.
The Catholic Church puts candidates through meticulous vetting.
In most cases the dossier must contain two "miracles". These are usually healings the church says are scientifically inexplicable and result from the candidate's posthumous intercession in answer to prayers.
Reputation for sainthood
Friends or relatives can apply posthumously for their loved one to be recognised as having a "reputation for sainthood", which gets the ball rolling on the full sainthood application process.
This usually begins at least five years after a person's death, although this was not the case for crowd favourites Mother Teresa and pope John Paul II for whom the timetable has been brought forward.
Once the candidate's holy reputation is recognised, he or she becomes a "Servant of God". A "postulator" is appointed to collect testimonies and compile all of the candidate's writings, producing a warts-and-all curriculum vitae.
The file then goes to the Congregation for the Causes of Saints, a Vatican department, where it is submitted to severe scrutiny by an official known today as the Promoter of Justice: formerly the Devil's Advocate.
If the Vatican gives the go-ahead, the candidate is henceforth considered "Venerable". The search begins for an inexplicable healing attributed to the candidate, who is considered to have interceded with God on behalf of the sick to bring about the "miracle".
Doctors and theologians are involved in in-depth investigations of such supposed miracles, and many candidates are rejected at this stage because scientific explanations cannot be ruled out.
If Vatican investigators recognise a "miracle", the candidate is "beatified", to be referred to henceforth as "blessed".
Many years can pass between the attribution of a first and a second miracle and some sainthood candidates never move beyond the "blessed" stage.
If a second miracle is recognised, however, the candidate can qualify for canonisation, becoming a saint.
In the case of "Good Pope" John XXIII, who launched the Vatican II reform drive in 1962, Pope Francis waived this requirement with the consent of the College of Cardinals.
The process can be very expensive, costing up to €725,000, and this has tended to favour candidates from the developed world. Italy boasts 260 saints, while only a dozen have come from Africa.
Pope Francis has however changed the rules to make the process less costly.
Other pathways to sainthood include martyrdom, with dying for one's faith warranting beatification, although a miracle is still required to attain sainthood.
Another means, known as "equivalent canonisation", recognises a candidate who has long been venerated as a saint in his or her local region. Pope Francis has created several saints on this basis.
In July 2017 the pontiff added a new pathway to becoming a saint for Christians who lay down their lives to save others.