21 In order that the Christian people may more certainly derive an abundance of graces from the sacred liturgy, holy Mother Church desires to undertake with great care a general restoration of the liturgy itself. For the liturgy is made up of immutable elements divinely instituted, and of elements subject to change. These not only may but ought to be changed with the passage of time if they have suffered from the intrusion of anything out of harmony with the inner nature of the liturgy or have become unsuited to it. In this restoration, both texts and rites should be drawn up so that they express more clearly the holy things which they signify; the Christian people, so far as possible, should be enabled to understand them with ease and to take part in them fully, actively, and as befits a community. Wherefore the sacred Council establishes the following general norms: A) General norms
Source: Sacrosanctum Concilium (Vatican.va)