In his book, Did Jesus Exist?, Bart Ehrman summarizes the historical evidence that "there really was a historical Jesus, a Jewish teacher who lived in Palestine as an adult in the 20s of the Common Era, crucified under Pontius Pilate sometime around the year 30." I've tried to summarize his summary, which one can find in his book, beginning on page 171:
"Among the Gospels we have numerous independent accounts that attest to Jesus's life, at least seven of them from within a hundred years of the traditional date of his death....They are based on written sources -- a good number of them -- that date much earlier, plausibly in some cases at least to the 50s of the Common Era....[The written sources] were based on oral traditions that had been in circulation year after year among the followers of Jesus....some of them...can be located in Jesus's homeland, Palestine, where they originally circulated in Aramaic. It appears that some, probably many, of them go back to the 30s CE. We are not, then dealing merely with Gospels that were produced fifty or sixty years after Jesus's alleged death as the principal witnesses to his existence. We are talking about a large number of sources, dispersed over a remarkably broad geographical expanse, many of them dating to the years immediately after Jesus's alleged life, some of them from Palestine itself. On the basis of this evidence alone, it is hard to understand how Jesus could have been 'invented.' Invented by whom? Where? When? How then could there be so many independent strands of evidence?
But that is just the beginning. The reality is that every single author who mentions Jesus -- pagan, Christian, or Jewish -- was fully convinced that he at least lived. Even the enemies of the Jesus movement thought so; among their many slurs against the religion, his nonexistence is never one of them....It is the view of all of our authors, for example, the authors of the epistles written both before and after Mark, whose views are based not on a reading of the Gospels but on traditions completely independent of Mark. It is also the view of Q and M and L and John and of all of John's sources. It is the view of the first-century books or letter of I Clement, I Peter, I John, Hebrews -- you name it. And it is also the view of the book of Acts, which preserves very primitive traditions in many speeches...that appear to date from the earliest years of the Christian movement, even before the followers of Jesus maintained that he was the Son of God for his entire life or even just from his baptism; according to these traditions, he became the son of God at his resurrection. This is the earliest Christology of them all, probably that of the original followers of Jesus, and so stems from the earliest Palestinian Christian communities. Once again we are back in the 30s of the Common Era, and the witness of these sources is unambiguous that Jesus existed.
The same results obtain by a careful study of Paul's letters. Paul came to know about Jesus within just a year or, at most, two of his death. Paul too preserves traditions that stem from the early period of his Christian life, right after his conversion around 32-33 CE. There is no doubt that Paul knew that Jesus existed. He mentions Jesus's birth, his Jewish heritage, his descent from David, his brothers, his ministry to Jews, his twelve disciples, several of his teachings, his Last Supper, and most important for Paul, his crucifixion. Paul indicates that he received some of these traditions from those who came before him, and it is relatively easy to determine when. Paul claims to have visited with Jesus's closest disciple, Peter, and with his brother James three years after his conversion, that is, around 35-36 CE. Much of what Paul has to say about Jesus, therefore, stems from the same early layer of tradition that we can trace, completely independently, in the Gospels."