To clarify for those who haven't studied the Bible, the earliest known Christian writings consist of Paul's Epistles and the Four Gospels. The Epistles are the surviving letters written by Paul to various groups that discuss matters relating to Christianity. Paul is considered the the "first Christian". The Four Gospels are written by Matthew, Mark, Luke, and John. These are the documents that give an account of the life of Jesus (ie. these books are where you'll find the stories of his birth, the sermon on the mount, the miracles he performed, his death, etc.) Additionally, Matthew and Luke aren't "independent" - they both borrow from writings in Mark's gospel (and possibly a 2nd source, look for the "Q Document" on wikipedia). Mark and John are the only two "independent" gospels. The gospels aren't completely cohesive (they contain contradictions as well as duplications).

Actual "historical" accounts of Jesus are simply not there. Again, look on wikipedia for the "historical Jesus" and you'll find that there are about 2-4 accepted historical (secular, non-religious) accounts of the man Jesus in the first century.

Furthermore, here is the chronology of the writings (c. means circa):

c. 5 CE - Jesus is born
c. 30 CE - Jesus is crucified
c. 49 CE - First new testament writing (One of Paul's Epistles)
c. 65 CE - Hypothesized "Q" source is written
c. 70 CE - The first known gospel (Mark) is written

As you can see, the first surviving document about the life of Jesus was written 30 years after his death.

This is not speculation, it is all fact, or at least is accepted as fact by the majority of scholars in the field.