Ehrman reaches all the way back to Homer and Plato to find out what thinkers thought about Heaven and Hell. In those days it was a matter of whether the Soul or the body was more important.
Around two hundred years before Christ the Jewish faithful were angry. Israel and Judah were being increasingly punished because sinners among the faithful had made God angry. The Babylonian Captivity really took the cake. The faithful wanted sinners, like the Roman emperors, punished. Previously Jewish holy men had little to say about the afterlife other than Sheol, a kind of nothing happening place the dead went to which wasn't much fun at all.
Jesus was all about the Judgement Day where the faithful would live on Earth, only an Earth without disease, wars or mosquitos. Sinners would be flushed, done away with. No eternal torture.
We can blame Paul for the idea that you had to accept Jesus as your Lord and Savior in order to inherit Super Earth.
Still Christian martyrs wanted to see sinners suffer and increasingly we moved toward a Heaven in the sky and eternal torture for sinners like Nero, who turns out to be the 666 mentioned in Revelation.
When Constantine converted to Christianity, the Romans were no longer the bad guys, so those who wanted revenge turned their eyes on their fellow Christians who were sinners.
Near the end of the book, Ehrman addresses Reincarnation and the thinker Origen who argued that everyone would be saved. After all God loved his children. Why would he subject any of them to eternal damnation and torture? But they would have to go through some kind of Boot Camp, such as Purgatory in order to be saved.
This is not an easy book to read, mainly because Erhman limits himself to the Heaven and Hell thing, and he repeats himself a lot. I thought the spooky stuff in Revelation would be fun, but somehow Erhman made it boring. But we did find out that the anti-Christ was Nero and that the Beast was the Roman Empire. Oh, yeah, most of these guys, including Paul, thought that the Messiah would come again in their own lifetimes or shortly thereafter. Ehrman isn't the same ornery self he was in JESUS INTERRUPTED, though. He doesn't say the Rapture is stupid, but he might as well have.