Theists: What is the most compelling argument you have heard for Atheism?

[Fake Messiah]

Not to mention the huge red herring we're expected to chase - nobody denies that xtians exist. But there's no way to get from there to the existence of a christ figure without blatantly palming a card, any more than we could establish Santa from the existence of his fanclub.

Christianity did exist, but it was very small in 1 CE. Later on it did grow but it was not a Christianity, but many denominations of Christians hating and killing each-other over which Christianity is real. So you have Roman writer Ammianus wrote in 4th century: "No wild beasts are so hostile to mankind as are most of the Christians in their savagery toward one another."

Christians were always like that, religious intolerance and disharmony were the norm for the better part of our history and they would be like it now, but the only reason  we see apparent unity today between conservative Catholics and conservative Protestants is because today they have common enemy which is secularism - they are no longer able to fight about whose Bible will be used or whose prayers will be said publicly, instead they joined forces to fight those who prefer no public prayers be said at all.
Also don't doubt that doctrinal disputes and prejudices between the two factions would quickly arise if the common secular enemy were removed.