We can distinguish three stages in the formation of the Gospels:
1. the life and teaching of Jesus. the Church holds firmly that the four Gospels, “whose historicity she unhesitatingly affirms, faithfully hand on what Jesus, the Son of God, while he lived among men, really did and taught for their eternal salvation, until the day when he was taken up.”
2. the oral tradition. “For, after the ascension of the Lord, the apostles handed on to their hearers what he had said and done, but with that fuller understanding which they, instructed by the glorious events of Christ and enlightened by the Spirit of truth, now enjoyed.”
3. the written Gospels. “The sacred authors, in writing the four Gospels, selected certain of the many elements which had been handed on, either orally or already in written form; others they synthesized or explained with an eye to the situation of the churches, the while sustaining the form of preaching, but always in such a fashion that they have told us the honest truth about Jesus.”
The fourfold Gospel holds a unique place in the Church, as is evident both in the veneration which the liturgy accords it and in the surpassing attraction it has exercised on the saints at all times: There is no doctrine which could be better, more precious and more splendid than the text of the Gospel. Behold and retain what our Lord and Master, Christ, has taught by his words and accomplished by his deeds.
But above all it’s the gospels that occupy my mind when I’m at prayer; my poor soul has so many needs, and yet this is the one thing needful. I’m always finding fresh lights there; hidden meanings which had meant nothing to me hitherto.
From the Catechism of the Catholic Church 126-127