"Lo, I am with you always, to the close of the age" (Mt 28:20)
…The question: "Teacher, what good must I do to have eternal life?" arises in the heart of every individual, and it is Christ alone who is capable of giving the full and definitive answer. The Teacher who expounds God's commandments, who invites others to follow him and gives the grace for a new life, is always present and at work in our midst, as he himself promised: "Lo, I am with you always, to the close of the age" (Mt 28:20). Christ's relevance for people of all times is shown forth in his body, which is the Church. For this reason the Lord promised his disciples the Holy Spirit, who would "bring to their remembrance" and teach them to understand his commandments (cf. Jn 14:26), and who would be the principle and constant source of a new life in the world (cf. Jn 3:5-8; Rom 8:1-13).
The moral prescriptions which God imparted in the Old Covenant, and which attained their perfection in the New and Eternal Covenant in the very person of the Son of God made man, must be faithfully kept and continually put into practice in the various different cultures throughout the course of history. The task of interpreting these prescriptions was entrusted by Jesus to the Apostles and to their successors, with the special assistance of the Spirit of truth: "He who hears you hears me" (Lk 10:16). By the light and the strength of this Spirit the Apostles carried out their mission of preaching the Gospel and of pointing out the "way" of the Lord (cf. Acts 18:25), teaching above all how to follow and imitate Christ: "For to me to live is Christ" (Phil 1:21).
In the moral catechesis of the Apostles, besides exhortations and directions connected to specific historical and cultural situations, we find an ethical teaching with precise rules of behaviour. This is seen in their Letters, which contain the interpretation, made under the guidance of the Holy Spirit, of the Lord's precepts as they are to be lived in different cultural circumstances (cf. Rom 12-15; 1 Cor 11-14; Gal 5-6; Eph 4-6; Col 3-4; 1 Pt and Jas). From the Church's beginnings, the Apostles, by virtue of their pastoral responsibility to preach the Gospel, were vigilant over the right conduct of Christians, just as they were vigilant for the purity of the faith and the handing down of the divine gifts in the sacraments. The first Christians, coming both from the Jewish people and from the Gentiles, differed from the pagans not only in their faith and their liturgy but also in the witness of their moral conduct, which was inspired by the New Law. The Church is in fact a communion both of faith and of life; her rule of life is "faith working through love" (Gal 5:6).
No damage must be done to the harmony between faith and life: the unity of the Church is damaged not only by Christians who reject or distort the truths of faith but also by those who disregard the moral obligations to which they are called by the Gospel (cf. 1 Cor 5:9-13). The Apostles decisively rejected any separation between the commitment of the heart and the actions which express or prove it (cf. 1 Jn 2:3-6). And ever since Apostolic times the Church's Pastors have unambiguously condemned the behaviour of those who fostered division by their teaching or by their actions.
Within the unity of the Church, promoting and preserving the faith and the moral life is the task entrusted by Jesus to the Apostles (cf. Mt 28:19-20), a task which continues in the ministry of their successors. This is apparent from the living Tradition, whereby—as the Second Vatican Council teaches—"the Church, in her teaching, life and worship, perpetuates and hands on to every generation all that she is and all that she believes. This Tradition which comes from the Apostles, progresses in the Church under the assistance of the Holy Spirit". In the Holy Spirit, the Church receives and hands down the Scripture as the witness to the "great things" which God has done in history (cf. Lk 1:49); she professes by the lips of her Fathers and Doctors the truth of the Word made flesh, puts his precepts and love into practice in the lives of her Saints and in the sacrifice of her Martyrs, and celebrates her hope in him in the Liturgy. By this same Tradition Christians receive "the living voice of the Gospel", as the faithful expression of God's wisdom and will.
Within Tradition, the authentic interpretation of the Lord's law develops, with the help of the Holy Spirit. The same Spirit who is at the origin of the Revelation of Jesus' commandments and teachings guarantees that they will be reverently preserved, faithfully expounded and correctly applied in different times and places. This constant "putting into practice" of the commandments is the sign and fruit of a deeper insight into Revelation and of an understanding in the light of faith of new historical and cultural situations. Nevertheless, it can only confirm the permanent validity of Revelation and follow in the line of the interpretation given to it by the great Tradition of the Church's teaching and life, as witnessed by the teaching of the Fathers, the lives of the Saints, the Church's Liturgy and the teaching of the Magisterium.
In particular, as the Council affirms, "the task of authentically interpreting the word of God, whether in its written form or in that of Tradition, has been entrusted only to those charged with the Church's living Magisterium, whose authority is exercised in the name of Jesus Christ". The Church, in her life and teaching, is thus revealed as "the pillar and bulwark of the truth" ( 1 Tim 3:15), including the truth regarding moral action. Indeed, "the Church has the right always and everywhere to proclaim moral principles, even in respect of the social order, and to make judgments about any human matter in so far as this is required by fundamental human rights or the salvation of souls".