The Agatha Christie indult is a nickname applied to the permission granted in 1971 by Pope Paul VI for the use of the Tridentine Mass in England and Wales. Indult is a term from Catholic canon law referring to a permission to do something that would otherwise be forbidden.
Following the introduction of the Mass of Paul VI to replace the Tridentine Mass in 1969-1970, a petition was sent to the Pope asking that the Tridentine form of the Roman Rite be permitted to continue for those who wished in England and Wales. Some English Roman Catholics had an attachment to the Tridentine Mass, as the Mass which had been celebrated by the English Martyrs of the Reformation and by priests in the years in which Catholicism had been subjected to sometimes severe persecution. However, the petition noted the exceptional artistic and cultural heritage of the Tridentine liturgy, and was signed by many prominent non-Catholic figures in British society, including Agatha Christie, Vladimir Ashkenazy, Kenneth Clark, Robert Graves, F. R. Leavis, Cecil Day-Lewis, Nancy Mitford, Iris Murdoch, Yehudi Menuhin, Joan Sutherland and two Anglican bishops, those of Exeter and of Ripon.
Cardinal John Heenan approached Pope Paul VI with the petition and asked that use of the Tridentine Mass be permitted. On 5 November 1971, the Pope granted the request. Supposedly Paul had read the letter and exclaimed "Ah Agatha Christie!" and so decided to grant the request; giving the indult its nickname. Between then and the granting of the worldwide indult in 1984, the bishops of England and Wales were authorized to grant permission for the occasional celebration of Mass in the old form, with the modifications introduced in 1965 and 1967.
The signatories of the original appeal to Pope Paul VI were: