Christianity is based on pagan religion
The idea that Christianity is based on pagan religion (Christ myth theory) comes from the 1790's works of Constantin-François Volney and Charles François Dupuis. While both said that Christianity was based on pagan religions they differed in the existence of Jesus as an actual human being. Dupuis held that there was no human being involved in the New Testament account which he saw as an intentional extended allegory of solar myths while Volney allowed for confused memories of an obscure historical figure to be integrated into a mythology that compiled organically.
The Christ myth theory would continue along these parallels line of thought up to the present day.
In fact, most Christ Mythers of the late 19th to early 20th century (such as Arthur Drews, John M. Robertson, James George Frazer, and John Remsburg) didn't dismiss the possibility that an actually human preacher had been involved in the creation of Christianity but also said the Gospels told us nothing real about that man (putting them in the Volney camp). Frazer and Remsburg both accepted the idea that Jesus had existed as human being and yet their ideas were called "Christ Myth".
The Dupuis camp hit rock bottom with Wheless' 1930 Forgery in Christianity which portrayed Christianity as this huge Illuminati sized conspiracy.
Obsolete or inaccurate arguments
Most modern Christ Myth theories borrow from Volney and Dupuis and throw in various amounts of Wheless. Because much of their information has been refuted (even by later Christ Mythers) this immediately kicks the whole argument in the head. The principle culprit of this is Graves' 1875 book The World's Sixteen Crucified Saviors 
Much is made of Justin's Dialogue of Justin and Trypho (the Jew) (69-70) where he said:
- "Be well assured, then, Trypho, that I am established in the knowledge of and faith in the Scriptures by those counterfeits which he who is called the devil is said to have performed among the Greeks; just as some were wrought by the Magi in Egypt, and others by the false prophets in Elijah's days. "
Yes it is nonsensical but it isn't the smoking gun some people make it out to be. It explains a sub set of Christianity not the whole movement and as Against Heresies (c. 180 CE) shows there were many divisions of Christianity.
Horus is said to be born of a virgin, when he was born (in one telling of the story) of Hathor and sex with the reassembled body of Osiris. Hathor already had other children. Another variant points to the fact Isis was merged with version Greek/Roman deities like Artemis/Diana
- As Irenaeus documents there was a sect of Christianity that "represented Jesus as having not been born of a virgin, but as being the son of Joseph and Mary according to the ordinary course of human generation"
- Paul in Romans 1:1-3 states that Jesus came "from the seed of David, according to the flesh" (the belief at the time was that women were the earth into which men planted their seed so here Paul expressly states that Jesus link to David is through the male line ie Joseph) and in Galatians 4:4 stated “God sent his Son, born of a woman” using the word gune (woman) rather than parthenos (virgin). Both these points show that Paul not only did not know of a virgin birth but expressly denied it.
- Virgin birth seems to be the ancient equivalent of being born with a silver spoon in one's mouth and signified the "extraordinary personal qualities exhibited by an individual" as well as being an "attempt to explain an individual's superiority to other mortals. Generally Mediterranean peoples looked at one's birth or parent-age to explain one's character and behavior" and "veneration of a benefactor." Caesar Augustus, Alexander the Great, Plato were all stated as being born of virgins and we know they were actual historical people; so the term 'born of a virgin' was never meant to be taken literally.
Mithras is said to be born of a virgin and to have died and been resurrected.
- Richard Carrier points out that Mithras is not a dying and rising god though he was a god of salvation.
"For when they tell that Bacchus, son of Jupiter, was begotten by intercourse with Semele, and that he was the discoverer of the vine; and when they relate, that being torn in pieces, and having died, he rose again, and ascended to heaven; and when they introduce wine into his mysteries" 
The birthday in common with pagan religions.
- In reality this was a very late comer to the story of Jesus. The December 25 date was by Emperor decree to compete with the popular Sol Invictus worship and first appears on a Roman calendar in 334 CE. Luke tells us that shepherds were tending their sheep in the fields when Jesus was born, something that shepherds did June until November. In fact, before the decree there was much debate regarding when Jesus was born. Tertullian (c 160–220 CE) and Hippolytus (c 170-235 CE) said March 25; Clement (c 150-215 CE) gave 25th day of Pachon (May 20) and the 24th or 25th of Pharmuthi (April 19 or 20), while others were saying January 6 (the birthday of Osiris), and still others pointed to the Essenes whose couples had sex in December so their child would be born September (the holy month of Atonement). 
- This means any argument that Jesus was a myth based on the December 25 date is doomed from the start because that part of the story isn't even in the Bible and didn't appear until well into the 4th century.
Lord Raglan hero pattern
Jesus's biography fits Lord Raglan's hero pattern remarkably well, with Jesus having a score of 18 to 20 out of 22. This makes him comparable with several legendary heroes, like Romulus and King Arthur (both 19). Well-documented heroes typically score much lower; Lord Raglan could not find real person with a score greater than 6 or 7. 
However, depending on how one uses the various points of Lord Raglan's hero pattern this is not true...even when Lord Raglan wrote his work in 1936:
Alexander the Great (356 - 323 BCE): 9 (Raglan gave him a 7)
Julius Caesar (100 - 44 BCE): 9 1/2
Augustus Caesar (63 BCE - 14 CE): 10
Napoleon Bonaparte (1769 - 1821): 8
Nicholas II, Tsar of All the Russias (1868 – 1918): 14
He we have three ancient known historical people and two modern known historical people who clock in well above the 7 claimed and it looks like Raglan may have fudged things to get Alexander the Great to fit his premise.
- ↑ Wells, G. A. "Stages of New Testament Criticism," Journal of the History of Ideas, volume 30, issue 2, 1969.
- ↑ Kersey Graves and The World's Sixteen Crucified Saviors (2003)
- ↑ 3.0 3.1 3.2 Ante-Nicene Fathers (1885) "Chapter LXIX.—The devil, since he emulates the truth, has invented fables about Bacchus, Hercules, and Æsculapius."
- ↑ Against Heresies Book 1 chapter 26 paragraph 1
- ↑ "Mary Was a Virgin" Jesus Police (Internet Archive)
- ↑ Tucker, Bob (2009) "Was Jesus Virgin Born?" Foundation of Contemporary Theology- Internet archive
- ↑ Talbert, Charles H. (2006) “Miraculous Conceptions and Births in Mediterranean Antiquity.” Pp. 79-86 in A.J. Levine, D.C. Allison and J. D. Crossan (eds.), The Historical Jesus in Context. Princeton: Princeton Univ. Press
- ↑ Richard Carrier, Why I Think Jesus Didn't Exist: A Historian Explains the Evidence That Changed His Mind
- ↑ John James Bond (1887) Bond's Handy-book for Verifying Dates pg 22
- ↑ "Born on December 25th" Jesus Police (Internet Archive)
- ↑ Hero Pattern
- ↑ http://www.religioustolerance.org/chr_jcpa4.htm Life events shared by Yeshua (Jesus) and the "Mythic Hero Archetype" ReligiousTolerance.org
- ↑ The Hero, pp. 184-85