It is true that there are no extra-biblical references to Jesus contemporaneous with his life. But what is being assumed without argument is that a criterion such as this is in fact a good one. Even Jeffrey Lowder, founder and past President of infidels.org, admits that it is not (see his artice “Independent Confirmation and the Historicity of Jesus” on infidels.org). As it happens, the Bible alone provides at least seven sources testifying to his existence, all of which are demonstrably independent of one another. Once again, it must be remembered that “the New Testament” is a collection of books written by different individuals at different times for diverse audiences in diverse ways. That they should all agree to the extent that they do with respect to the life of Jesus is truly remarkable. The sources are as follows:
(1) The Gospel of Mark – recognized by most to be the earliest of the gospels.
(2) The Gospel of John – considered by nearly everyone to be independent of Mark, Matthew, and Luke.
(3) M – the material found only in Matthew (whether based on his own recollections or other sources)
(4) L – the material found only in Luke
(5) Q – the material common to Matthew and Luke that is absent from Mark. Most recognize this source as independent of the others, though this is disputed by some.
(6) Paul: he testifies to seeing Jesus himself, and is acknowledged by most to have written his letters before the gospels were written.
(7) 1 Corinthians 15:3-5 – Paul says that he received this particular formulation from others.
If we add to this the testimony of Josephus, who made two references to Jesus (one of which is normally regarded as at least partly spurious, though the second is nearly universally recognized as genuine), we have EIGHT sources. Anyone who denies the historicity of Jesus in the face of this body of evidence is obviously running into a fairly strong headwind.
You may also be interested in Did Jesus Exist? and Were Bible stories and characters stolen from pagan myths?