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

[SteveII]

So how does the NT help us establish the truth of the claims in the NT? Besides ad populum, I mean.

First, I would not say this is a case of ad populum. If my argument was solely based on "people believed", you would have a point. However, I am arguing that "people who were eyewitnesses and influenced by eyewitnesses believed". Now, of course not everyone was an eyewitness of the resurrection that belonged to those first churches, but they were alive during a time that rebuttal witnesses would have abounded. While you might say that any rebuttal witnesses might have been 'edited out' of history, we do have the evidence of the tremendous growth of the church from day 1. 

Regarding size of the church, we get a clue from Tacitus, who descibing Nero in 64AD and those arrested as "an immense multitude" in Rome. 64AD is still very much within the lifetime of eyewitnesses and rebuttal witnesses. 

Regarding the general interpreting of evidence as to who Jesus might really be? There is all kinds of evidence to weigh. 

- Documentary (both actual and inferred)
- The churches, the growth, the persecution, and the occasional mention in surviving secular works.
- The characters, their actions, character, stated goals, meaning of their words, and eventual circumstances
- Jesus' own claims (explicit, implicit, connections to the OT--some of which the disciples may have never known). 
- The actual message: how it seems to fit the human condition, resonate with people, and how it does not contradict the OT--which would have required a very sophisticated mind to have navigated that. 
- Paul and his writings on application--done before the Gospels were independently written. To have them work so well together is incredible. 
- This one can't be stressed enough: the likelihood of alternate theories to explain the facts. I think it is obvious people believed from day one when Jesus was still walking around. I have never heard a alternate theory which could account for most or all of the concrete and circumstantial evidence we have. 

You could write books on any one of the points above (and people do). The point is, it is not as simple as saying "there is no evidence" There are layers upon layers of evidence that one person or another will find somewhere between uninteresting to compelling.