The Christological argument for the existence of God is a relatively modern argument. It is an indirect argument based on the claims of Jesus Christ. That is, if one accepts that Jesus existed, that the Biblical account of Jesus is largely true, that Jesus' claims are valid, and that Jesus claims God exists, one should accept God exists.
Modern evangelism often takes this approach. Potential converts are introduced to Jesus as a historical character and the merits of Jesus's teachings are discussed. In such a context, the historicity of Jesus of Nazareth naturally takes on enhanced urgency; the usual historian's questions of documentation, authentication, and the like, tend to be removed from ordinary historical discourse, to take supportive places within Christological theology.
Interestingly, many of the tenets of this approach were under fire before it was even put to paper. Bertrand Russell, in his famous essay, Why I am not a Christian, criticized Jesus' person character and philosophical positions on various grounds.
