He is only the third pope to be given this honor. Millions of Catholics are expected to visit the basilica to pay homage to John XXIII, one of the most respected pontiffs of the 20th century.
The embalmed body of Blessed John XXIII, who died on Pentecost Sunday, June 3, 1963, had since his death been in the crypt, alongside the remains of dozens of other popes. The remains of many other popes are buried in the interior of the basilica.
The only other popes in glass coffins for public viewing are Blessed Innocent XI (who died in 1689, and was beatified by Pius XII in 1956) and St Pius X (who died in 1914, was beatified in 1951 and canonized in 1954 by Pius XII).
When John XXIII's coffin was opened 38 years after his death, his body was practically intact. "A miracle," some Italians declared, but without support from the Vatican authorities.
Gennaro Goglia, who in 1963 was a professor of anatomy at the Catholic University of the Sacred Heart in Rome, also rejected suggestions that there had been a miracle.
Dr Goglia told the daily newspaper Famiglia Cristiana that he injected Pope John's body with a "special liquid" to preserve human remains which had been developed by Professor Winkler of the University of Lausanne (Switzerland), an "authority in this field."
Before Sunday's ceremony, Blessed John's body was clothed in pontifical vestments and placed in a 450-kilogram bronze-and-glass coffin. On Pentecost Sunday, June 3, the coffin was carried in procession to St Peter's Square, ...