Julius Africanus argues that the genealogies of Jesus in Matthew and Luke are consistent with each other, and he cites historical witnesses to that effect (Letter To Aristides, 5). He comments that he's going to provide "the true history of these matters" (1). Regarding the darkness at the time of Jesus' crucifixion, Julius comments that the event is recorded in a history written by Thallus (see fragment 18:1 from his Chronography here). Tertullian and Jerome also appeal to non-Christian sources for corroboration of the darkness. If those Christians thought the gospel accounts of the darkness were fictional, why would they be appealing to non-Christian sources, non-fictional sources like a book of history and government records, for corroboration?
Origen refers to "the history recorded in the Gospels" and refers to how enemies of Christianity interpreted Jesus' birth in Bethlehem in the same historical manner as he does:
"With respect to the birth of Jesus in Bethlehem, if any one desires, after the prophecy of Micah and after the history recorded in the Gospels by the disciples of Jesus, to have additional evidence from other sources, let him know that, in conformity with the narrative in the Gospel regarding His birth, there is shown at Bethlehem the cave where He was born, and the manger in the cave where He was wrapped in swaddling-clothes. And this sight is greatly talked of in surrounding places, even among the enemies of the faith, it being said that in this cave was born that Jesus who is worshipped and reverenced by the Christians." (Against Celsus, 1:51)
Origen often refers to his own historical interpretation of the New Testament in his treatise Against Celsus, and he frequently refers to corroboration of that view from non-Christian sources. Celsus wants a reliable historical witness to support the gospels' account of the descent of the dove at Jesus' baptism (1:41), and Origen's reply discusses what's involved in "substantiating...historical fact" (1:42) and "historical probability" (1:44). Origen notes that Celsus is arguing that the gospel account is "fictitious" (1:43). Notice that Celsus thinks he's arguing against Christianity by charging the gospels with fiction, and Origen thinks that he as a Christian should defend the historicity of the account.
Like other early Christians, Origen contrasts pagan myths that are fictional and the New Testament accounts of Jesus' life that are supported by "much evidence" (1:67). Pagan mythology is to be rejected as unhistorical, whereas the New Testament's presentation of Jesus is supported by the testimony of eyewitnesses and their willingness to suffer for their testimony (3:23).
There are many other such passages in Against Celsus and elsewhere in early Christian literature. Robert Wilken is correct:
"The question of the mythological and legendary character of the Gospels did not first arise in modern times. The historical reliability of the accounts of Jesus' life was already an issue for Christian thinkers in the second century....The question of faith and history, so much a part of modern theological discourse since the Enlightenment, was also a significant part of the debate between pagans and Christians in the ancient world....Already in the second century, however, Celsus devoted part of his True Doctrine to a critical examination of the accounts of Jesus' life, and Porphyry paid even greater attention to the literary and historical analysis of the Scriptures....The primary issue in the debate over the Bible was whether the Scriptures could be considered a reliable source for the words and events they record....Pagan critics realized that the claims of the new movement [Christianity] rested upon a credible historical portrait of Jesus. Christian theologians in the early church, in contrast to medieval thinkers who began their investigations on the basis of what they received from authoritative tradition, were forced to defend the historical claims they made about the person of Jesus. What was said about Jesus could not be based solely on the memory of the Christian community or its own self-understanding." (The Christians As The Romans Saw Them [New Haven and London: Yale University Press, 1984], pp. 112, 147, 203)