The main evidence used by Christians to prove the canonicity of the Bible is that there is an extensive number of manuscripts or both the Old and New Testaments, but actually this is not an enough evidence, as the number is not the only factor that tells if it is true or not, what is more important is the time these manuscripts were written, for example in case of the Old Testament, the two major manuscripts are Masoretic text and the Septuagint. The Masoretic text refers back to the ninth century, while the oldest manuscript for Septuagint refers back to the fourth century. As for the New Testament, the oldest manuscripts are the Syriac Peshitta referring to about 200AD, while contains the New Testament except 2 Peter, 2 John, 3 John, Jude and Revelation, also the Greek Vaticanus and Sinaiticus which are considered to be the main manuscripts used, and refer to the fourth century.
These are the oldest manuscripts for both the Old and New Testaments, as we see for the Old Testament, the oldest manuscript refers to the fourth century, and for the New Testament, the oldest manuscript refers to the beginning of the third century, and doesn't contain some epistles, which puts a question mark concerning them, were they added to the Bible? This makes a gap between the time of the Prophets including Jesus (Peace be upon him), and the time of writing these manuscripts, and makes the claim that the Bible was fully preserved not very accurate, because it could have happened that the writers of these manuscripts were anonymous.
Some might say that there were earlier fragments that prove that the books of the Bible were present at that time as the fragments of John Rayland for example which refers to about 125 AD, and has some words in the Gospel of John, but actually this is not a proof even for the existance of Gospel John at that time, all that it can prove is that these words were present at that time, but it could have been taken from another source, and it may have been that the writer of Gospel John copied it from that source,or..., many possibilities exist, but this is not an evidence in itself.
This is also the same case for the Dead Sea Scrolls, all what was found in Qumran were mostly fragments, the only book which was found to be nearly complete is Isaiah, but all other Old Testament books were fragments that doesn't prove that the Bible was present the same as it is now, especially that a lot of Apocrypha were found in Qumran, which puts another question mark concerning the books which used to be canonical at that time, and on what base were the books of the Bible taken as canonical and others were not? especially that a lot of these apocrypha were present with the canonical books in the same manuscript as 1-4 Maccabees and the Prayer of Manasseh which are present in the Vaticanus with other canonical books.
Also another evidence used to prove that the Bible was preserved is the quotes of the Early Church Fathers, but actually the first Early Church Fathers who extensively quotes from the Bible lived at the end of the second century as Irenaeus and Tertullian, those who were before them as Ignatius, Polycarp, Barnabas and Justin Martyr quoted very few verses from the Bible especially the New Testament, may be Justin Martyr quoted a lot from the Old Testament, but his quotes from the New Testament were very few, besides, a large portion of these few quotes were not really quotes, but they were only narrations by meaning, what I mean is that it could have been that a common source from where the Gospels copied, and there is already a theory telling that there was a common source for the Synoptic Gospels called the Q source, and many theologians adopt this theory. You can make sure of what I am saying concerning the quotes of the Church Fathers by downloading E-sword program from e-sword.net and installing the book of Ante-Nicene Fathers on the program, it will be highlighting the scripts, and you can easily distinguish them with no need to read all these books.
Besides, many Early Church Fathers had heretical opinions, for example Theophilus of Antioch said in his book "To Autolycus" Chapter 15 that the Tinity is "God, His word and His Wisdom", and in Chapter 10 that the Word is the Spirit of God. Papias says " Judas walked about in this world a sad example of impiety; for his body having swollen to such an extent that he could not pass where a chariot could pass easily, he was crushed by the chariot, so that his bowels gushed out. ", which doesn't match at all with what the Gospels say, Ireneous who said that Jesus was more than 50 yrs when he died, Tertullian who was following the heretical Montanus, and Origen whom Philip Schaff tells concerning him:"For — and in this too he is like Schleiermacher — he can by no means be called orthodox, either in the Catholic or in the Protestant sense. His leaning to idealism, his predilection for Plato, and his noble effort to reconcile Christianity with reason, and to commend it even to educated heathens and Gnostics, led him into many grand and fascinating errors.", and there are other examples, but I only wanted to give examples. All this proves that the Christian belief wasn't settled at that time, which gives the conclusion that the real Gospels were not that genuine, and that a lot of problems happened at that critical time.