Authors such as Tom Harpur, suggest that not only was Jesus based on already existing pagan god-men, but the early church was well aware of this. According to Harpur and others, it was only much later that the church forgot their mythic origins and mistakenly accepted the story of Jesus as historical.
In responding to this, it is helpful to look at the Church Fathers. One of the early Fathers was Theophilus of Antioch who lived AD 115-181. In chapter 9 of his first book to Autolycus, Theophilus contrasts Christianity with the pagan myths. You can judge for yourself if this reveals any early church knowledge of Jesus being based on pagan myths.
“And, indeed, the names of those whom you say you worship, are the names of dead men. And these, too, who and what kind of men were they? Is not Saturn found to be a cannibal, destroying and devouring his own children? And if you name his son Jupiter, hear also his deeds and conduct— first, how he was suckled by a goat on Mount Ida, and having slain it, according to the myths, and flayed it, he made himself a coat of the hide. And his other deeds—his incest, and adultery, and lust—will be better recounted by Homer and the rest of the poets. Why should I further speak of his sons? How Hercules burnt himself; and about the drunk and raging Bacchus; and of Apollo fearing and fleeing from Achilles, and falling in love with Daphne, and being unaware of the fate of Hyacinthus; and of Venus wounded, and of Mars, the pest of mortals; and of the ichor flowing from the so-called gods. And these, indeed, are the milder kinds of legends; since the god who is called Osiris is found to have been torn limb from limb, whose mysteries are celebrated annually, as if he had perished, and were being found, and sought for limb by limb. For neither is it known whether he perished, nor is it shown whether he is found. And why should I speak of Atys mutilated, or of Adonis wandering in the wood, and wounded by a boar while hunting; or of Æsculapius struck by a thunderbolt; or of the fugitive Serapis chased from Sinope to Alexandria; or of the Scythian Diana, herself, too, a fugitive, and a homicide, and a huntress, and a passionate lover of Endymion? Now, it is not we who publish these things, but your own writers and poets.”