R.L. Fox is a renowned historian just to mention one but the point is moot: being a theologian (or better, in the case of many of those mentioned, a New Testament scholar) does not invalidate your research, as long as it has been conducted with proper methodology. Inevitably, studying historical figures has you going where the sources are and, in the case of Jesus, a lot of those (but not all) come from the Bibles; I would doubt any scholar who completely disregard such an important source: you may question some passages or the veracity of certain events but not even looking at it?

Furthermore, we have ample proofs from non-christian sources that recognise Jesus very existance: from Tacitus to Josephus, from Mara to the very Talmud, and it's recognised as a prophet even by Islam. The thing is, the biblical Jesus, the one that performed miracles and was resurrected after three days, might well have never existed; the historical Jesus, the one that, in the worst case scenarion, was a trickster who eventually founded a religion that would last for thousands of years, did.