The fact is, one doesn't need to "believe in" Jesus to believe that he actually existed. No less a scholar than Bart Ehrman, former evangelical turned agnostic and arch rival of conservative scholars, has come out in support of the existence of Jesus as a historical person.
In a recent article on the Huffington Post Craig Keener has also come out in support of a historical Jesus. Craig is a good historian and outlines some of the evidence we have for Jesus while at the same time dismissing the claims of the mythicists. Here is some of what he says.
Contrary to some circles on the Internet, very few scholars doubt that Jesus existed, preached and led a movement. Scholars' confidence has nothing to do with theology but much to do with historiographic common sense. What movement would make up a recent leader, executed by a Roman governor for treason, and then declare, "We're his followers"? If they wanted to commit suicide, there were simpler ways to do it.
One popular objection is that only Christians wrote anything about Jesus. This objection is neither entirely true nor does it reckon with the nature of ancient sources. It usually comes from people who have not worked much with ancient history. Only a small proportion of information from antiquity survives, yet it is often sufficient.
We recognize that most people write only about what they care about. The only substantive early works about Socrates derive from his followers. The Dead Sea Scrolls extol their community's founder, but no other reports of him survive. The Jewish historian Josephus claims to be a Pharisee, yet never mentions Hillel, who is famous in Pharisees' traditions. Israeli scholar David Flusser correctly observes that it is usually followers who preserve what is most meaningful about their teachers, whether the leaders were Buddha, Muhammad, Mormon leader Joseph Smith or African prophet Simon Kimbangu.
You can read the whole article here.