so what those so called historians and theologians use to prove the existence of Jesus?
Bart Ehrman (who is himself a secular agnostic) wrote in his 2011 review of the state of modern scholarship
Forged : writing in the name of God: "He [Jesus] certainly existed, as virtually every competent scholar of antiquity, Christian or non-Christian, agrees." Which I would think would be a book right up your alley.
Robert E. Van Voorst in his 2000
Jesus Outside the New Testament: An Introduction to the Ancient Evidence states: "biblical scholars and classical historians regard theories of non-existence of Jesus as effectively refuted" He also points out that in antiquity, the existence of Jesus was never denied by those who opposed Christianity (and if they could they certainly would have).
John Dominic Crossan (a name that anyone who wants to dabble in this should recognize) writes in his
Jesus: A Revolutionary Biography. "That he [Jesus] was crucified is as sure as anything historical can ever be, since both Josephus and Tacitus...agree with the Christian accounts on at least that basic fact."
Michael Grant (who is a classicist) wrote in his 2004 book
Jesus "In recent years, 'no serious scholar has ventured to postulate the non historicity of Jesus' or at any rate very few, and they have not succeeded in disposing of the much stronger, indeed very abundant, evidence to the contrary."
Geoffrey Blainey writes that by the standards of the late first century B.C. and early first century A.D. Jesus' life is "astonishingly documented" – more so than any of his contemporaries – with numerous books, stories and memoirs written about him. The problem for historians is not therefore, determining whether Jesus actually existed, but rather in considering the "sheer multitude of detail about his life and its inconsistencies and contradictions."