The double canonisation on Sunday will be presided over by Pope Francis in St Peter's Square of the two men he believes revitalised the Church while giving it purpose and direction.
The current pontiff described John Paul II and John XXIII as "wonderful" and "brave men", and bent years of tradition by allowing them to be celebrated on the same day.
A candidate for sainthood would normally have to pass a rigorous test which begins at least five years after their death and includes the verification of two miracles.
John Paul II had his canonisation fast-tracked (as he did for Mother Theresa during his pontificate) while John XXIII candidacy was pushed through without having to clear the hurdle of a second confirmed miracle.
John, who was born Angelo Roncalli, reigned between 1958 and 1963 and is credited with saving the lives of thousands of Jews during the Second World War.
He gave apostolic visas to many Jewish people fleeing Nazi persecution in eastern Europe, and also ended centuries of enmity between religions by rejecting anti-Semitism in the Second Vatican Council which modernised the Roman Catholic Church.
He is also famous for the so-called 'Speech of the Moon' in 1962 when he appeared on the balcony of the Vatican and told the faithful to go home and "hug their children," telling them it came from the Pope.
Some Vatican scholars say the Council, known as Vatican II, caused a rift in the Church between traditionalists and modernisers, which remains to this day.
One of those who decried the attempt to change the Church's stance on a number of fundamental issues was Karol Wojtyla, who became John Paul II in 1978.
John Paul II's canonisation has been fast-tracked
Seen as the most influential Pope of modern times, John Paul made 104 apostolic journeys around the world in his 27 years at the Vatican and created 482 saints of his own.
Vatican scholars believe this double canonisation is an attempt by Francis to heal the Church, while also showing his deep gratitude to the two popular leaders, both from humble origins, who influenced his theology and teachings.
The logistics of the event are extraordinary: 2,500 volunteers were on hand for the morning mass, with 17 giant screens broadcasting the service to those unable to pack into the Square.
On Saturday night a number of prayer vigils will be held across central Rome, with confessions heard in a variety of different languages.
It is estimated one million people will take part in the event.
There has been criticism that the swift canonisation of John Paul II ignores criticism that he failed to tackle the growing problem of priest sex abuse, especially in the United States.
The Vatican says an "information gap" between church officials in America and the Vatican was behind the speed of the response, which meant he was not "living the crisis in real time".