According to Catholic weekly the Tablet, 5 million pilgrims are expected in Rome for the canonisation of the two popes on Sunday.
The Tablet explains that normally, two miracles attributed to the deceased need to be verified before they can become saints.
The first for John Paul II was the curing of a French nun, said to be suffering with Parkinson’s. The second was a Costa Rican woman, cured of a brain aneurism.
In deciding to sanctify Pope John XXIII, the church made an exception with one miracle.
Most of the world’s 1.2bn Catholics will agree that these two men, in their own ways, were holy and charismatic pastors who helped their 2,000-year-old church to confront the challenges of the modern era.
However, the role of saints within and outside the Catholic faith has often been subject to debate.
Some say saints offer hope to millions of Catholics around the world – but other say the phenomenon has become too commercial.
James Roberts, assistant editor at the Tablet, told Channel 4 News: “Sainthood emphasises the connection between the divine world and what we’re living.
“The process of canonisation helps link the divine to what’s happening on earth.
“If you’re in a faith you accept one can pray for intersession, you believe that prayers are answered.”
Popular way of devotion
Relics are also an important part of the Catholic faith. A relic usually consists of the physical remains of a saint or the personal effects of the saint or venerated person.
Mr Roberts added: “Relics are a popular way of devotion. [Many people] pray before a relic because they are both material and spiritual.”
But according to Kristina Cooper, editor of the UK’s Catholic Charismatic Renewal (CCR) magazine Good News, “if we return to the early church, then we’re all called to be saints.”
Speaking to Channel 4 News, Ms Cooper said: “Some people you meet are saints as soon as you meet them – and they aren’t always a nice people.”
Ms Cooper added: “Lots of people are saints, but they aren’t investigated to the same extent [as John Paul II and John XXIII]”.