St. Hippolytus wrote in a piece that is historically "accepted as genuine that Jesus was born nine months after the creation of the world, a date which the early fathers identified as March 25. Thus we have a clear reference to Jesus' birth on December 25 before there ever was established the cultic celebration of the sun god in Rome," Zia said.
Barber added that there are other reasons to believe that it was in fact the pagans who were copying the Christians, in this and other ways.
"Justin Martyr (in First Apology 66) accuses the followers of Mithras of imitating the eucharistic practice of the Church," he said.
The Church fathers' understanding of March 25 as the date of the creation of the world is also a key to understanding the theological view of why Christmas is on December 25, said Fr. Michael Witczak, associate professor of liturgical studies and sacramental theology at The Catholic University of America in Washington, D.C.
As the understood date of the creation of the world, March 25 is also fittingly celebrated as the feast of the Annunciation, or the day that Mary conceived Jesus by the power of the Holy Spirit, he said.
"December 25 is related to March 25. So March 25th is the celebration of the feast of the Annunciation...and March 25th is nine months before December 25," Witczak said.
Witczak added that the Church does not know for certain that these were the exact dates on which these events occurred, as these exact dates are not mentioned in Scripture, and we have few early historical documents on them. Even the year thought to be that of Jesus' birth may be off by a few years, he said.
But understanding Jesus as an historical person who actually existed, and understanding him in his divinity, are two different things, he added.
"I don't think any good historian would doubt the historicity of Jesus Christ or Saint Paul," Witczak said, "but knowing that Jesus was a historic figure (versus) believing that he's the second Person of the blessed Trinity...it's a matter of faith."
Even if December 25 as a date for the celebration of Christmas were taken from a pagan tradition, Witczak said, it does not negate the person of Christ.
If December 25 was originally a festival of a sun god, for example, "as a Christian, I would say, but the divinity is not the sun, the divinity is the creator God who created the world and everything in it. This is revelatory of the presence of God in our life and in our world," he said.
"When Pope Gregory The Great sent St. Augustine to England to bring the faith to the British people, he gave him advice about how to consecrate pagan temples to use as a Christian Church," Witczak said.
"In Mexico, there are some aspects of the way that they celebrate All Souls Day, the Dia de los Muertos, that are reminiscent of Aztec customs. The image of Our Lady of Guadalupe, she's dressed in Aztec clothes and the belt that she wears is the belt that a pregnant woman would have worn in pre-Christian days in the Aztec world," he said.
"So this dynamic of taking over things from a previous culture and then using it for Christian purposes...it's kind of part and parcel of the way that the Church operates," he said.
Zia said the claims of strong links between Jesus and paganism tell of a lack of catechesis among believers, and a lack of faith among non-believers.
In reality, "there is ample room to include pre-Christian elements in the celebration of our Christian holidays" Zia said, and "the core purpose of each specific Christian holiday is utterly new and not a borrowing from pagan holidays."
Zia encouraged anyone who was really interested in the parallels between Jesus and any number of pagan gods to do their own research, and not rely on the claims of books such as Acharya's.
"(D)epending on how much time and effort one puts into the task, one can find occasional parallels between Jesus and just about any pagan deity from any culture," Zia said.
"Yet such comparisons are too few and too weak to suggest that Jesus is simply the 'Christian' version of these former pagan beliefs. When we look at the entire context of the identity, life, mission, and teachings of Jesus, then it becomes abundantly clear that he is utterly unique in the history of the world," he said. "It is for this reason that Jesus – and only Jesus – could boldly proclaim, 'Behold, I make all things new!'"