Today we live in a world far more advanced than that of our forefathers thousands of years ago.  Most of us (except those live in under-developed countries or remote places) have access to excellent information systems.  News and events from all over the world can reach us within a short time through media like TV, radio, newspaper and the Internet, and it can be documented and stored for further reference.  Speeches or statements and even physical appearances made by leaders and other dignitaries can be recorded word-by-word or pixel-by-pixel, then stored or copied and transmitted to any part of the world within seconds.  A number of choices for storing the information, whether it is text, sound or graphics, are available.  Paper in separate sheets or bound into a book is still the most common information storage in written form.  But we also have microfilms, video, audiocassette, diskette, compact and hard disk from our personal computers that serve the same purpose.  Any information can be exactly duplicated and transmitted easily.  We can photocopy and send by mail or fax any information printed on paper.  Modern printing machines can produce huge amounts of books and newspaper within a short time.  We can copy any file in our personal computer and have it sent through e-mail. Once our personal computer is connected to the Internet, we can view and download tonnes of information.  While to a certain extent, other than radio and TV, we still use oral transmission (words of mouths) of information, we tend to rely less on it and may even label it as rumour, unreliable and even a misleading source of information.  We prefer written or graphical information and some of us may even demand it to be issued by a reliable institution.  In addition, most of us are literate in, at least, one language that gives us access to the information.  Most, if not all languages in the world can be put in written form using its own script or a script borrowed or developed from other scripts.  Chinese, Arabic and Greek have their own script but English and many languages in the world must use twenty-six characters developed from Latin or Roman characters.

The situation was significantly different even a few hundred years ago; let alone thousand of years ago.  Christians believe that God started communicating with us through the prophets thousand of years ago when the information system was still primitive.  It was still primitive even when our Lord Jesus and His apostles preached the good news of the kingdom of God around two thousand years ago.  The available information storage was clay tablets, stone (cf. Exodus 24:12, Joshua 8:32, 1 Kings 8:9, 2 Corinthians 3:3), bones, writing board (cf. Isaiah 30:8, Habakkuk 2:2, Luke 1:63), papyrus and parchment (cf. 2 Timothy 4:13) but no paper and obviously no books.  There was no TV, no newspaper, no radio and no computer network to transmit information.  Obviously in such a situation the oral transmission played more of a role than that of today.

In fact, our Lord's (and the later apostolic) teachings were first transmitted and taught orally.  When Paul quoted Jesus' words in his epistles, the source was not any of the four Gospels (they were not written yet) but from what he heard from others or from Jesus directly (1 Corinthians 11:23).  He spent fifteen days with Peter - that could be one of his sources of information.  In 1 Corinthians 15:3-7, he wrote that what he delivered was what he received, which must have been through oral communication.  For example, the order to whom Jesus appeared after His resurrection (verses 5 to 7) is not recorded in any of the four Gospels, nor even in the Book of Acts.  The hymn he quoted in Ephesians 5:14 might have come from an early Christian hymn he heard.  Oral transmission of words might lead to paraphrasing.  Thus 1 Corinthians 7:10 echoes what Jesus said in Luke 10:7 and Jesus' words in Acts 20:35 might echo Luke 6:38 but both are not exact quotations.  The first Christians did not consider this oral transmission as inferior. In Galatians 4:20, Paul wrote that he wished to be with the Galatians, so they could hear his tone.  Because that was impossible, then a letter from him would be sufficient. On the other hand, in 2 Corinthians 1:23-2:4 Paul judged that a written communication would be more effective than anything he could say. In 1 Corinthians 11:34, he insisted on meeting the Corinthians to give directions on other matters.   Similarly, in 2 John 12, the writer preferred to talk directly rather than using written letter.  The third Gospel (Luke 1:1-4) claims that the writer simply put in written form what the eyewitnesses and ministers of the word (orally) delivered to him.

The word "tradition" (Greek "paradosis") is the noun of the verb (Greek "paradidomi") that means "to deliver" or "to hand over".  Note that Greek was the language in which the New Testament books were written.  The New Testament testifies that the Word of God may be handed down in both written and oral forms.

In Philippians 4:9, Paul asked the Philippians to do what they had learned, received, heard and seen in him.  Based on the above verses, the Catholic Church teaches that the Gospel (here means the good news, the teaching of Jesus and of the apostle, not the four Gospels) was handed down in two ways: orally and in writing (CCC # 76).  By "oral", the Church means by the apostles who handed on, by their spoken word of their preaching, by example they gave, by institutions they established, what they themselves had received, whether from the lips of Christ, from his way of life and works, or whether they had learned it at the prompting of the Holy Spirit.   In "writing", this means that the apostles and others associated with them (like Mark and Luke) who, under the same inspiration of the same Holy Spirit, put the message in written form.  Thus, the Church teaches that the Bible is the Word of God put down in writing under the breath of the Holy Spirit (2 Timothy 3:16) and that the (unwritten) Holy Tradition transmits in its entirety the Word of God given to the apostles by Christ and the Holy Spirit (CCC # 81).  As a result the Church derives Her certainty about revealed truths from both the Bible and Tradition (CCC # 82).  Does the New Testament support this teaching?  The New Testament never claims it contains all the teachings of Jesus and the apostles.  In fact, it claims otherwise, e.g. there are other things Jesus did that was not put in written form (John 20:30, 21:25).  If Jesus had intended to make Christianity a religion of the book exclusively, He would have then prepared and made sure that all His teachings were carefully written and preserved.  The four Gospels do not record any occasion that Jesus gave an order to put His teachings in written form.  In Revelation 1:19, He asked John to write down only whatever he saw, which comes to us as the Book of Revelation, not the whole Gospels.  Among the apostles we have only books written by Peter, John and Paul and two books from two brothers James and Jude.  The oral teaching of the other apostles was not put in written form and there is no guarantee that it was included in what Peter, John, Paul, James and Jude wrote.  Luke, being a companion of Paul wrote mostly on the activities of Paul and not of other apostles.  We have also the testimonies from the Church Fathers about the existence of (oral) tradition.  For example, Papias (c. 60-130), bishop of Hierapolis (in present day Turkey), who knew Matthew and Mark Gospels preferred oral tradition to written one.  Obviously, he chose so because as the first generation of Christian he still had the chance to meet some of the apostles.

Irenæus (c. 115 to 202), bishop of Lyon wrote (English translation is from Anti Nicene Fathers, Vol. 1):

But, again, when we refer them [Gnostics] to that tradition which originates from the apostles, [and] which is preserved by means of the succession of presbyters in the Churches, they object to tradition, saying that they themselves are wiser not merely than the presbyters, but even than the apostles, because they have discovered the unadulterated truth. For [they maintain] that the apostles intermingled the things of the law with the words of the Saviour; and that not the apostles alone, but even the Lord Himself, spoke as at one time from the Demiurge, at another from the intermediate place, and yet again from the Pleroma, but that they themselves, indubitably, unsulliedly, and purely, have knowledge of the hidden mystery: this is, indeed, to blaspheme their Creator after a most impudent manner! It comes to this, therefore, that these men do now consent neither to Scripture nor to tradition. ........

It is within the power of all, therefore, in every Church, who may wish to see the truth, to contemplate clearly the tradition of the apostles manifested throughout the whole world; and we are in a position to reckon up those who were by the apostles instituted bishops in the Churches, and [to demonstrate] the succession of these men to our own times; those who neither taught nor knew of anything like what these [heretics] rave about........

Since, however, it would be very tedious, in such a volume as this, to reckon up the successions of all the Churches, we do put to confusion all those who, in whatever manner, whether by an evil self-pleasing, by vainglory, or by blindness and perverse opinion, assemble in unauthorized meetings; [we do this, I say, ] by indicating that tradition derived from the apostles, of the very great, the very ancient, and universally known Church founded and organized at Rome by the two most glorious apostles, Peter and Paul; as also [by pointing out] the faith preached to men, which comes down to our time by means of the successions of the bishops. For it is a matter of necessity that every Church should agree with this Church, on account of its pre- eminent authority, that is, the faithful everywhere, inasmuch as the apostolical tradition has been preserved continuously by those [faithful men] who exist everywhere.

In this order, and by this succession, the ecclesiastical tradition from the apostles, and the preaching of the truth, have come down to us.  And this is most abundant proof that there is one and the same vivifying faith, which has been preserved in the Church from the apostles until now, and handed down in truth........

Since therefore we have such proofs, it is not necessary to seek the truth among others which it is easy to obtain from the Church; since the apostles, like a rich man [depositing his money] in a bank, lodged in her hands most copiously all things pertaining to the truth: so that every man, whosoever will, can draw from her the water of life.  For she is the entrance to life; all others are thieves and robbers. On this account are we bound to avoid them, but to make choice of the thing pertaining to the Church with the utmost diligence, and to lay hold of the tradition of the truth. For how stands the case? Suppose there arise a dispute relative to some important question among us, should we not have recourse to the most ancient Churches with which the apostles held constant intercourse, and learn from them what is certain and clear in regard to the present question? For how should it be if the apostles themselves had not left us writings? Would it not be necessary, [in that case,] to follow the course of the tradition which they handed down to those to whom they did commit the Churches?

Irenaeus, Against Heresies 3.2.2, 3.3.1, 3.3.2, 3.3.3, 3.4.1 (emphasis added)