"The Christian ideal has not been tried and found wanting. It has been found difficult and left untried." --G.K. Chesterton
Far be it from us too deny, that we know what we have learned by the testimony of others: otherwise we know not that there is an ocean; we know not that the lands and cities exist which most copious report commends to us; we know not that those men were, and their works, which we have learned by reading history; we know not the news that is daily brought us from this quarter or that, and confirmed by consistent and conspiring evidence; lastly we know not at what place or from whom we have been born: since in all these things we have believed the testimony of others. And if it is most absurd to say this, then we must confess, that not only our own senses, but those of other persons also, have added very much indeed to our knowledge.
St. Augustine, On the Trinity, book 15, chap. 12
From the introduction to The Navarre Bible: St. Mark, pp. 42-45 (Scepter Press), emphasis added:
Many episodes in Jesus' life are so concrete and so tiny in the framework of world history, that it is very difficult to check them by referring to non-Christian sources: in fact even the Gospels themselves provide few such checks. This does not mean that they are not historically checkable, i.e. that it is not theoretically possible to check them, but that in practice it is very difficult to find any historical documentation to support them, outside the Gospels themselves, In cases like this, as in so many others in world history, historical science can assess at least the degree of credibility which such accounts merit by critical examination of the testimonies themselves. Historians tend to require (i) that the authors be sincere, i.e. that they are striving to tell the truth; and (ii) that they be well-informed on the matters in question. If careful study shows that these or similar requirements are met then the document is historically acceptable. Well, in the case of the Gospels these requirements are in fact met.
(i) The human authors of the Gospels did write in order to pass on to us the truth about what they had seen and heard (cf. Lk 1:1-4; Jn 21:24). Although they did not write their histories in exactly the same way as modern historians (for example, as far as chronological and topographical exactness goes) they still really did try to tell the facts just as they happened. Thus in the case of St. Matthew and St. Luke, when they tell about the temptations Christ underwent in the desert, they give them in a different order but both of them tell us what really happened, It is also true that the evangelists were enlightened by the Holy Spirit when they wrote and that their purpose was the strengthen the faith of their readers: but that did not lead them to falsify events or neglect historical accuracy; on the contrary: the very articles of our faith include some which are historical facts, such as those to do with Christ's life (his Incarnation, Death, Resurrection etc.), which is why St. Paul asserts that if Christ had not risen then our faith would be in vain (cf. 1 Cor 15:14, 17). Hence the evangelists do what they set out to do precisely by telling the true story of what happened.
However, each evangelist, under the inspiration of the Spirit, tries to lay special emphasis on particular aspects of Jesus and his work: and to this end each plans his book differently. The evangelists, even those who were witnesses of Jesus' public life, each remember things differently depending on their mentality and the circumstances in which they are operating. The Holy Spirit does help them to remember and to understand everything they have seen and heard (Jn 14:16) but this does not cancel out their human faculties or the use of memory. Also, each evangelist drew on different sources, and these sources had peculiarities of their own--thus explaining the different order in which they narrate words and events in our Lord's life, and the differences which we find between Gospel and Gospel. But this does not in any way take away from their historicity; on the contrary, as St. Augustine explains, ``it is quite likely that the evangelists believed they had a duty to tell things in the order in which God suggested them to their memory, at least in matters where the order in which they were reported in no way took from the truth and authority of the Gospel. For the Holy Spirit distributes gifts to each individually as he wills (1 Cor 12:11). He directed and ruled the minds of the saints [the sacred writers] to ensure that the books would have full authority' in bringing to their minds the things they should write, the Holy Spirit would allow each to plan his narrative as he thought fit and in such a way that anyone who read it carefully and devoutly would be able to read it with God's help.'' 
Another proof of the evangelists' sincerity is their fortitude in bearing persecution, even to the point of giving their lives to bear witness to the truth they pass on to us in their writings. From what has been said, it is clear that the human authors of the Gospels were sincere and strove to tell the truth.
(ii)But investigation into the historicity of the Gospels need to go further than that: were the writers well-informed about the subject in question, or were they, unintentionally mistaken or misled? In reply to this question we must say that, humanly speaking, they were very well informed, because they have been eye-witnesses of the vents they describe or had been in contact with eye-witnesses. We should also add that the educational method used by Jesus--which was similar to rabbinical method of the time--consisted in repeating the same teachings time and again, to ensure his disciples got a good grasp of them. This learning method, based on exercising memory, meant that the Apostles could remember many sayings of Jesus which the evangelists later put down in writing. the fact that they sometimes do not pass on exactly the same wording must be due to (a) the fact that Jesus said these things often, on different occasions and in different ways in the course of his preaching in various parts of Palestine; (b) the evangelists being more concerned about getting the meaning right than giving his exact words; (c) differences naturally arising in translations from the original into Greek.
Another factor which gives us the guarantee that the evangelists were not mistaken is that their narratives coincide in essentials with the preaching of St. Peter after Pentecost, as reported in the book of the Acts of the Apostles. There are no grounds, therefore, for thinking that the Gospel story was the product of the fertile hyper-imagination of the first generation of Christians: from the very first moment, fifty days after the Resurrection, the very same things were already being proclaimed, and the Apostles, in their zeal to perform the mission of preaching entrusted to them by our Lord, were extremely careful not to pass on anything which was not true. A proof of this is that the Church never accepted as authentic later outpourings of popular imagination such as the apocryphal gospels.
The evangelists, then, were well-informed about the material they were with; and the fact that their books were received by the Church as divinely inspired indicated that their content concurred with what the Apostles, in their preaching about Jesus, had spread all over the world. It is true that the evangelists do not present our Lord's life in the way a modern biography would; but they do give us reliable information about what Jesus, the Son of God, taught and did when he was living among us. They do not tell us everything about Christ, but they have left us a permanent and divine witness to the truth we need to know about our Lord and his teaching.
After the meticulous research the Gospels have been subjected to over the past two centuries, any serious critic must accept that the sacred books are true history. The main difficulty which some people experience is the fact that supernatural phenomena, the miracles of Jesus, appeared frequently in the Gospels. Ultimately this involves questions of faith: either one accepts the supernatural or one does not. So in fact we find that one of two things happen: if a person accepts the supernatural and God's direct intervention in human and physical events, that is, the possibility of miracles, they key question of the historical truthfulness of the Gospels is solved in the affirmative; if a person does not accept the possibility of God directly intervening in history in this way, then from the outset he denies the historicity of the Gospels, because if one denies the possibility of miracles one cannot accept as true writings which report miracles even if these documents scientifically merit acceptance as being historical.
Summing up what we have said, we can assert that when our faith tells us that the Holy Gospels ``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 of his ascension,''  this faith is based on a solid, reasonable foundation; it is not a blind option; it is based on certain facts which are part of history and which have been passed down to us in reliable documents (the Gospels) in the bosom of a society which authenticates them (the Church), and which has faithfully conserved them and which offers us a correct interpretation of them. But these historical facts can only be deeply understood and fully accepted if one has the divine gift of faith. As St. Augustine puts it referring to people of his own time, who found some passages in the Gospels an obstacle to their conversion: ``Let him who asks these things become a Christian, lest in trying to solve all the questions about the holy books he should end his life before having passed from death to life. There are innumerable problems that cannot be solved before believing, at the risk of ending his life without faith. Once faith has been accepted, then they can be studied in detail as an exercise for the pious enjoyment of the faithful mind.''
1. De consensu Evangelistarum, 2, 21, 51
2. Dei Verbum, 19
3. Letter, 102, 6, 38
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