Christ sent his apostles so that “repentance and forgiveness of sins should be preached in his name to all nations”. “Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit.” The mission to baptize, and so the sacramental mission, is implied in the mission to evangelize, because the sacrament is prepared for by the word of God and by the faith which is assent to this word:
The People of God is formed into one in the first place by the Word of the living God…. the preaching of the Word is required for the sacramental ministry itself, since the sacraments are sacraments of faith, drawing their origin and nourishment from the Word.
“The purpose of the sacraments is to sanctify men, to build up the Body of Christ and, finally, to give worship to God. Because they are signs they also instruct. They not only presuppose faith, but by words and objects they also nourish, strengthen, and express it. That is why they are called ‘sacraments of faith.”‘
The Church’s faith precedes the faith of the believer who is invited to adhere to it. When the Church celebrates the sacraments, she confesses the faith received from the apostles – whence the ancient saying: lex orandi, lex credendi (or: legem credendi lex statuat supplicandi according to Prosper of Aquitaine [5th cent.]). The law of prayer is the law of faith: the Church believes as she prays. Liturgy is a constitutive element of the holy and living Tradition.
For this reason no sacramental rite may be modified or manipulated at the will of the minister or the community. Even the supreme authority in the Church may not change the liturgy arbitrarily, but only in the obedience of faith and with religious respect for the mystery of the liturgy.
Likewise, since the sacraments express and develop the communion of faith in the Church, the lex orandi is one of the essential criteria of the dialogue that seeks to restore the unity of Christians.
From the Catechism of the Catholic Church 1122-1126