I think what the Christian was basically trying to get accross the whole time was that in order for the atheist to say that he KNOWS God doesn’t exist, he would need to be all-knowing, which would make him God. If the atheist would have just come out and said that in reality it is possible for God to exist but he just doesn’t think it is likely, then that would have probably stopped a lot of the questioning by the Christian. It was like he didn’t want to even allow for the possiblility, which is absurd. At one point the atheist asks whether the Christian understands what atheists believe. He says that atheists “say there is no God.” But the atheist didn’t clarify whether he says there is no God because he knows it or because he thinks that is most likely.
The theist doesn’t need to be all-knowing in order for him to say that God does exist though. He doesn’t need the ability to peek into every crack and crevice of the universe. If God in reality were to communicate to him or visit him then he could be completely ignorant about everything but still know that God exists. Of course in one sense it is impossible for him to know that what he saw/heard/felt was indeed God because he could just be plugged into the Matrix or something, so he has to make some assumptions like the universe actually existing. The atheist would need to make the same assumptions too, because it is possible that he is plugged into a Matrix that is run by God.