Another objective of the Movement was the introduction of the vernacular language (in particular, into the Mass of the Catechumens, i.e. the part of the liturgy which includes the readings from the Bible). This, it was believed, would assist the congregation's spiritual development by enabling them to participate in the celebration of Mass with understanding. Pope Pius XII, who had a particular interest in the liturgy, wrote in his 1947 encyclical "Mediator Dei" that "the use of the mother tongue in connection with several of the rites may be of much advantage to the people", though he stated at the same time that only the Holy See had the authority to grant permission for the use of the vernacular. He granted permission for the use of local languages in the renewal of baptismal promises in the Easter Vigil service.