19 Holy Mother Church has firmly and with absolute constancy held, and continues to hold, that the four Gospels just named, whose historical character the Church unhesitatingly asserts, faithfully hand on what Jesus Christ, while living among men, really did and taught for their eternal salvation until the day He was taken up into heaven (see Acts 1:1). Indeed, after the Ascension of the Lord the Apostles handed on to their hearers what He had said and done. This they did with that clearer understanding which they enjoyed after they had been instructed by the glorious events of Christ's life and taught by the light of the Spirit of truth. The sacred authors wrote the four Gospels, selecting some things from the many which had been handed on by word of mouth or in writing, reducing some of them to a synthesis, explaining some things in view of the situation of their churches and preserving the form of proclamation but always in such fashion that they told us the honest truth about Jesus. For their intention in writing was that either from their own memory and recollections, or from the witness of those who "themselves from the beginning were eyewitnesses and ministers of the Word" we might know "the truth" concerning those matters about which we have been instructed (see Luke 1:2-4).
Source: Dei Verbum (Vatican.va)