The 'Christ myth theory' Wikipedia page actually provides quite an in-depth exposition on the development of the concept Jesus wasn't a historical person:
The beginnings of the formal denial of the existence of Jesus can be traced to late 18th century France, and the works of Constantin François Chassebœuf de Volney (1757–1820) and Charles-François Dupuis (1742–1809). Volney and Dupuis argued that Christianity was an amalgamation of various ancient mythologies and that Jesus was a totally mythical character.
In 1835, German theologian David Friedrich Strauß (1808–1874) published his extremely controversial The Life of Jesus, Critically Examined (Das Leben Jesu). While not denying that Jesus existed, he did argue that the miracles in the New Testament were mythical retellings of normal events as supernatural happenings.
German Bruno Bauer (1809–1882), who taught at the University of Bonn, took Strauss' arguments further and became the first author to systematically argue that Jesus did not exist.
A key component of the theory is the supposition that the new testament texts are utterly useless in providing any accurate historical data regarding 'the person of Jesus of Nazareth'.
However, most scholars today, even those with a critical view of the new testament, agree that Jesus certainly existed and made some manner of impact on the religious-political scene of first-century Judea. His crucifixion during the governorship of Pilate is a historical fact taken for granted, and his baptism by John is usually admitted even by those who criticize the reliability of the new testament in its depiction of historical events.