Pope Francis has invoked his "magisterial authority" to declare that the liturgical reforms following the Second Vatican Council are "irreversible". His statement came in a major 2,500-word address to Italian liturgists taking part in a national liturgy week. The Pope argued that the reforms did not "flourish suddenly" but had a long gestation. He cited measures taken by Pius X and Pius XII in the first half of the 20th century – Pius X creating a commission for renewal in 1913 and Pius XII with his 1947 encyclical Mediator Dei and changes to Holy Week liturgy. This culminated in the Second Vatican Council's constitution Sacrosanctum Concilium, which, he said, "desired a living liturgy for a Church completely vivified by the mysteries celebrated.” New liturgical books were promulgated, the Pope said, but there is "still work to do" in reforming people's mentality – "in particular rediscovering the reasons for the decisions made with the liturgical reform and overcoming unfounded and superficial readings, partial receptions and practices that disfigure it.” This did not mean “rethinking the reform", the Pope argued, but "knowing better the underlying reasons [for it]… [and] of internalising its inspirational principles and of observing the discipline that governs it.” Pope Francis has previously said that “to speak of ‘the reform of the reform’ is an error". When the Vatican's liturgy chief Cardinal Robert Sarah spoke favourably of the "reform of the reform" at a talk in London he was criticised by then Vatican spokesman Fr Federico Lombardi, who said it would be better “to avoid the use of the expression ‘reform of the reform' ... given that it’s sometimes the sources of misunderstandings.”