21 Liturgical practice begins with the very founding of the Church. The first Christians, in fact, "were persevering in the doctrine of the apostles and in the communication of the breaking of bread and in prayers." Whenever their pastors can summon a little group of the faithful together, they set up an altar on which they proceed to offer the sacrifice, and around which are ranged all the other rites appropriate for the saving of souls and for the honor due to God. Among these latter rites, the first place is reserved for the sacraments, namely, the seven principal founts of salvation. There follows the celebration of the divine praises in which the faithful also join, obeying the behest of the Apostle Paul, "In all wisdom, teaching and admonishing one another in psalms, hymns and spiritual canticles, singing in grace in your hearts to God." Next comes the reading of the Law, the prophets, the gospel and the apostolic epistles; and last of all the homily or sermon in which the official head of the congregation recalls and explains the practical bearing of the commandments of the divine Master and the chief events of His life, combining instruction with appropriate exhortation and illustration of the benefit of all his listeners.
Source: Mediator Dei (Vatican.va)