There are a few reasons why Christians are Christian. One of the main reasons is to do with Jesus and who he was and what he did. After all if he was who he said he was and performed those miracles, rose from the dead then it would be foolish to deny his claims about being the son of God and that the way to heaven is through him. What's a way to convince them that the gospels cannot be true?
Cool - good to know! It means there is hope!
Take the story, replace all references to "Jesus" with "Sponge Bob" and have the person read the story again.
If I might offer a suggestion, in defending Jesus people often point to witnesses (the women at the tomb) and things like the Trilemma. But these are all avoidances of the questions, not responses. If a Harry Potter religion started up, would you pose the Trilemma to Harry Potter, or to JK Rowling? Similarly, would testimony of 3 witnesses or hundreds of witnesses matter, if they all came from one person, JK Rowling?
There's a bottleneck in Christianity (and I've been meaning to write a blog post about this). Of the four gospels, two are essentially anonymous, one is a summery of the other three, and one comes indirectly from Simon Peter. The rest of the New Testament is mostly written by Paul, a man who never met Jesus.
So the details of Jesus (as opposed to detail-less mentions of a religious leader named Jesus) all come from a bottleneck of one person. Is it possible for one person to lie about Jesus? It doesn't matter if Simon Peter claims 3 or 300 witnessed the resurrection. It doesn't matter what Simon Peter claims Jesus claimed, if it's all just one person saying what other people said and did.
Even if we got all our information about Jesus from one person, that would still mean he's on par or above most other figures of his time. Consider that our best source on Jewish history, Flavius Josephus, is often the only one attesting to the existence of Jewish figures and rarely if ever met these figures in person.
But regardless, the four gospels are not reducible quite that easily. Mark is clearly drawing on an oral tradition that predates the gospel (as you can tell by the Aramaic quotes, for instance) and through Matthew and Luke we have access to at least the Q document, and possibly other oral traditions. John is most suspect in this context since it's the latest.
But that's quite a bit more than a one person bottleneck.
There have been many "spiritual gurus" through history, Jesus was probably one of them or an union of various myths.
Bart Ehrman's book "Misquoting Jesus" was influential in my deconversion. He is one of the foremost scholars on Christianity and can explain how mistakes occurred in the New Testament. It's a great book and hard to argue with without sounding openly ignorant.
The most common claim I see among non-theists is that, because the first gospel (Matthew) mentions the destruction of the Temple Mount, it must have been written after 70 C.E. Because it was not authored until a generation after Jesus's supposed death, it cannot be counted as a first-hand, accurate account of his life. That is not something I have looked into throughly, so I cannot atest to whether the claim is true.
However, the comparisons between Jesus and earlier deities always strike me. It makes me think that old stories were tacked onto a man who did exist.
You're slightly misremembering the argument.
Actually the gospel of Matthew is not the first gospel; that would have to be Mark. Matthew is most likely based on both Mark and the Q document (at least according to double source theory).
But the argument is actually not that it can't be written before 70 AD because there's a prophecy of the destruction/desecration of the Temple Mount; otherwise Christians would be right to say that we're simply assuming the gospel to be wrong in principle, and then making our conclusions based on that (besides, for my money it's rather likely that Jesus did indeed make this prophecy; the downfall of the Jewish leadership was something that featured prominently in Jewish apocalyptic thought, so this could likely be a historical element).
The actual part that's interesting is that the writer of Mark emphasizes this part of the story:
14But when ye shall see the abomination of desolation, spoken of by Daniel the prophet, standing where it ought not, (let him that readeth understand,) then let them that be in Judaea flee to the mountains:
The abomination of desolation seems to be a clear reference to the Roman flag (dedicated to the pagan Gods; abomination) being planted on top of the Temple of a pillaged Jerusalem (desolation). But since Mark is underlining this by making an explicit appeal to the reader, he must be aware that this has already happened. The Jewish-Roman war lasted from 67-70 AD, which means that Mark is written -at the very earliest- after 67 AD, but more likely slightly after 70 AD.
That's one of the ways we date Mark.
The part many atheists tack on at the end is less impressive: the idea that since it is not first-hand (which is true) it must therefore be discarded. This is a standard of quality that very, very few documents of the Ancient world can clear: most documents are not first-hand and non-contemporary... but that simply means we need to carefully analyse them (like any other source in history), not wholly discard them.
The fact that it's not first-hand is enough for us to discount miracles, but not to dismiss everything that's in there.
There are so many interpretations of the mythical prophet of the Christians, one in a concatenation of syncretizations of centuries of speculation about the sun and why there are seasons, in joyous lip service to what are often mistakenly thought his teachings, it is wise to keep in mind that no argument may be won that has dogma in its opposition. They will never understand the relationship between Revelations and Nero Caesar, nor will they cease thinking that Jesus was anti-abortion and wouldn't mind making money hand over fist at the enslavement of the working middle class. In Joel's mind, passing through the eye of a needle is merely a timely witticism; air conditioned dog houses are OK if you are a televangelist, and if the Bible's English was good enough for King James I, it is good enough for me. We are a long way from the original Koine Greek, which had no punctuation, such that the words "GOD IS NOW HERE" could also mean "GOD IS NOWHERE." Don't waste your breath, brother.
Simply point out the similarities between how christianity keeps its members and how modern day cults do.
My first line of attack would be that I view heaven and hell as quite obviously a control tactic, and thus this concept is obviously 'intelligently designed', by humans. For example, a clear comparison can be drawn between heaven and hell and the fact that the church of $ c i e n t o l o g y has its inner circle ($ e a 0 r g) sign a billion year contract to come back life after life for eternity to serve the church. To 'blow' or leave that organization means to give up one's 'eternity', to forever be declared as a '$uppressive person', and to "die over and over again". Point out to the lay christian that this tactic works for $ c i e n t o l o g y and prevents people from leaving the church, and often imprisoning people (literally) for decades, convincing them that everyone outside of the church is a criminal, homeless, living wretched lives and that any desire to leave will lead to such a life for the person deciding to give up on this religion. It's easier to see that a cult is controlling people if it's a cult that you don't believe in. Just simply drawing comparisons between an obviously ridiculous cult and that person's "not so obviously" ridiculous religion can really make someone question the validity of their beliefs. Unless they are the leader. (Then you make sure not to touch any doorknobs on your way out of the building)
Of course a person cannot break loose from the trap of heaven and hell until they are ready, and anyone who has made a serious time commitment as a member of the "inner circle" of the local christian cult will be much harder to convince. Deconversion is not an exact science. But in the right circumstances, it can work. Someone had to challenge my beliefs before I went through the thought process that led me to consider myself an atheist.
For a young person who is fed up of living in fear (of god or x e n u) and senses intuitively that there is something wrong with the belief system they've been indoctrinated into, it should be relatively easy if they are gently nudged in the right direction. If I met such a person today, I would recommend reading "A Portrait of the Artist as a Young Man" by James Joyce, as reading this book helped solidify in my mind that heaven and hell is a control mechanism. The description of hell in that book brought me to tears when I was 15. Once one sees that it is all a trap, the rest should fall into place gradually, as the person learns to cope with their new-found thought freedom. Of course, there are a million other variables, but the crux of why people are afraid to ask questions, especially when they are barely old enough to have processed the bible on their own, is the concept eternal damnation. It is a human invention, and just as destructive to humanity, in my opinion, as the atom bomb.
I got carried away and veered way off topic... Sorry!