I figured I'd revive this thread, as there's still plenty of debating left to do, IMO.
Let's take this one point at a time.
My argument:
1: The new testament as we have it today is in the same form as it was originally written nearly 2000 years ago.
Proof of 1:
This is fairly simple, as the new testament has a ridiculous number of manuscripts supporting it from all over the place in multiple languages, with some fragments dating all the way back to early second century. To focus on one specific type, there are 306 unical manuscripts (written in all-caps greek letters), which date back to as early as the third century. The most important of these would be the Codex Sinaiticus, which is a complete copy of the new testament (the only one written in this form, actually) which dates to about 350 AD, long before the catholic church would have been doing any significant meddling. As earlier texts agree with it, I think it's fair to say that we have the New Testament as it was originally written. (As a side note: The total number of ancient manuscripts would be around 24,000, far more then we have of any other ancient documents.)
2: Non-biblical evidence about Jesus.
From A Case for Christ, by a historical expert: (slight paraphrasing here)
Even discarding every scrap of the new testament or other Christian writings, we would still know the following about him: First, Jesus was a Jewish teacher. Second, many people believed that he performed miracles. Third, that some people believed he was the Messiah. Fourth, that he was rejected by the Jewish leaders. Fifth, that he was crucified under Pontius Pilate in the reign of Tiberius. Sixth, that despite this shameful death, his followers, who believed he was still alive, spread beyond Palestine so that there were multitudes of them in Rome by AD 64 (about 30 years after his death). Finally, that all kinds of people (man, woman, slave, free, from all sorts of locations) worshiped him as God.
Proof of 2:
Let's start with the recordings of Josephus. Josephus was a Jewish historian, a priest, and a Pharisee, therefore making him part of the group of Jesus's harshest critics. (Despite this, he was very unpopular with the Jews, as he collaborated with the hated Romans.) His most ambitious work was called "The Antiquities", as a history of the Jews from Creation up until his time. It was completed around 93 AD.
Here's the quote:
"He [Ananias, a high priest] convened a meeting of the Sanhedrin and brought before them a man named James, the brother of Jesus, who was called the Christ, and certain others. He accused them of having transgressed the law and delivered them up to be stoned."
This proves the existence of Jesus, and it mentions that Jesus was called the Christ. This is an important point, as Christ means "The Annointed One", or "Messiah". Therefore, some people believed Jesus was the Messiah. As far as I know, NOBODY has successfully debated this passage. (Note the neutral historical tone and a delivery of the bare facts. If there was some Christian addition later on, wouldn't you expect it to be supportive of James?)
Josephus also wrote an even lengthier section about Jesus, which is called the Testimonium Flavianum (which is also REALLY hotly disputed, by the way.)
Here's the quote:
"About this time there lived Jesus, a wise man, if indeed one ought to call him a man. For he was one who wrought surprising feats and was a teacher of such people as accept the truth gladly. He won over many Jews and many of the Greeks. He was the Christ. When Pilate, upon hearing him accused by men of the highest standing among us, had condemned him to be crucified, those who had in the first place come to love him did not give up their affection for him. On the third day he appeared to them restored to life, for the prophets of God had prophesied these and countless other marvelous things about him. And the tribe of Christians, so called after him, has still to this day not disappeared."
For obvious reasons this passage is controversial. (Note how supportive it is of Jesus, compared to the former's cold delivery of the facts.) Scholarship has gone through three trends about it. For obvious reasons, early Christians loved it as a corroboration of Jesus's lifetime. Then the entire passage was questioned by some scholars during the Enlightenment. However, today there's a remarkable consensus among both Jewish and Christian scholars that the passage as a whole is authentic, although there may be some interpolations. (That is, early Christian copyists inserted some phrases that a Jewish write like Josephus wouldn't have written.) For example, the first line says, "About this time there lived Jesus, a wise man." That is likely authentic, as it isn't normally the way Christians would speak about Jesus, The next phrase says "if indeed one ought to call him a man.", implying Jesus is more than human, and therefore likely an interpolation. The other two points that are likely interpolations would be the two unambiguous statements "He was the Christ", and "On the third day he appeared to them restored to life." Considering how elsewhere Josephus doesn't make these sort of absolute "Christianity is TRUE" sort of statements, you can see how they were likely added later. However, even when you take those out, you end up with the info that Jesus was the martyred leader of the church in Jerusalem, and that he was a wise teacher who managed to make a large and long-lasting following despite his crucifixion at the hands of Pilate and the request of the Jewish leaders.
Anyway, despite these and other writings, we still have the question, "Why isn't there more backing outside of the gospels?". However, the answer is fairly simple, and has to deal with the persecution of the first-century church.
Let me start with a quote from Tacitus, a Roman historian.
"Nero fastened the guilt and inflicted the most exquisite tortures on a class hated for their abominations, called Christians by the populace. Christus, from whom the name had its origin, suffered the extreme penalty during the reign of Tiberius at the hands of one of our procurators, Pontius Pilatus, and a most mischievous superstition, thus checked for the moment, again broke out not only in Judaea, the first source of the evil, but even in Rome.... Accordingly, an arrest was first made of all who pleaded guilty: then, upon their information, an immense multitude was convicted, not so much of the crime of firing the city, as of hatred against mankind."
You see, there was immense persecution of anyone who called themselves a Christian back then. Therefore, people would obviously be either for Christ, against him, or be very careful to have nothing to do with Christianity! In the first case, any significant testimony would count as Christian (and possibly added to the Bible), which we're ignoring for the moment. In the other two cases, would you write significant testimony supporting a figure whose cause was under intense persecution? You might as well put a "Crucify me" sign on your back.
Additionally, Jesus gives people very few options with his claims to divinity. You either have to dismiss him as a lunatic (As C.S. Lewis put it, "On the level of a man who claims he's a poached egg"), condemn him as a liar, or worship him as Lord. Therefore, any testimony about him by it's very nature, would have to either confine itself to the bare facts (and be strictly historical, such as the one by Josephus), condemn him (as the Talmud does, an important work by the Jewish community), or confirm him as Lord (which would therefore be Christian, which we're ignoring for the moment.)
Finally, the same exact question can be applied to any other significant event in ancient history, and time and time again, it turns out that the events of Jesus's lifetime have far more backing then most.
3: The New Testament itself. I claim that this gives an accurate representation of Jesus's life and teachings.
Proof of 3:
Firstly, in all likelihood the authors of the New Testament (not to mention the thousands of other Christians from that time) suffered intense persecution and death for their beliefs. Let me quote another historical source, Pliny the Younger. (This is from a letter he sent to Emperor Trajan.)
"I have asked them if they are Christians, and if they admit it, I repeat the question a second and third time, with a warning of the punishment awaiting them. If they persist, I order them to be led away for execution; for, whatever the nature of their admission, I am convinced that their stubbornness and unshakable obstinacy ought not to go unpunished....