“Christ the Lord, in whom the entire Revelation of the most high God is summed up, commanded the apostles to preach the Gospel, which had been promised beforehand by the prophets, and which he fulfilled in his own person and promulgated with his own lips. In preaching the Gospel, they were to communicate the gifts of God to all men. This Gospel was to be the source of all saving truth and moral discipline.”
In the apostolic preaching. . .
In keeping with the Lord’s command, the Gospel was handed on in two ways:
– orally “by the apostles who handed on, by the spoken word of their preaching, by the example they gave, by the institutions they established, what they themselves had received – whether from the lips of Christ, from his way of life and his works, or whether they had learned it at the prompting of the Holy Spirit”;
– in writing “by those apostles and other men associated with the apostles who, under the inspiration of the same Holy Spirit, committed the message of salvation to writing”.
From the Catechism of the Catholic Church 75-76