From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
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,35 just as they were vigilant for the purity of the faith and the handing down of the divine gifts in the sacraments.36 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.37 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.38
35 Cf. Saint Irenaeus, Adversus Haereses, IV, 26, 2-5: Sources Chrétiennes (SCh) 100/12, 718-729.
36 Cf. Saint Justin, Apologia, I, 66: Patrologiæ Cursus completus, Series Græca (PG) 6, 427-430.
37 Cf. 1 Pt 2:12ff; cf. Didache, II, 2: Patres Apostolici, ed. F. X. Funk, I, 6-9; Clement of Alexandria, Pædagogus, I, 10; II, 10: PG 8, 3ff-364; 497-536; Tertullian, Apologeticum, IX, 8: Corpus Scriptorum Ecclesiasticorum Latinorum, 69, 24.
38 Cf. Saint Ignatius of Antioch, Ad Magnesios, VI, 1-2: Patres Apostolici, ed. F. X. Funk, I, 234-235; Saint Irenaeus, Adversus Hæreses, IV, 33:1, 6, 7: SCh 100/ 2, 802-805; 814-815; 816-819.
— Veritatis Splendor 26