Bart D. Ehrman, Heaven and Hell: A History of the Afterlife (New York: Simon & Schuster, 2020), 192-193.
As it turns out, it is possible to trace a trajectory in our surviving Gospels away from the deeply apocalyptic teachings of Jesus in Mark and Matthew, to less apocalyptic teachings in the later Gospel of Luke, to non-apocalyptic teachings in the still later Gospel of John, to anti-apocalyptic teachings in the noncanonical Gospel of Thomas, written a couple of decades after John. In short, the words of Jesus, over time, came to be de-apocalypticized.
Jewish apocalyptic thought is essentially dualistic, stressing not only that there are two fundamental components of reality – good and evil, God and the devil – but also that all of history can be divided into two ages, the current age that will be destroyed and the future age in which God will reign supreme. This is a kind of “horizontal” dualism in that you can map it out on a time line across the page from left to right. When Christians de-apocalypticized the teachings of Jesus, they retained a dualistic understanding of the future but they flipped the temporal, horizontal dualism on its axis so that it became a vertical conception, not moving from left to right but from below to above. The emphasis now is not on time – this age and the age to come – but on space: this awful world on earth and the glorious world above in heaven. It is no longer about “now and then” but about “down and up.”
This new conception is thus still dualistic, but rather than emphasizing God’s kingdom to come in the future it proclaims God’s kingdom now to be enjoyed in the world above. Everyone who sides with God will go to that Kingdom of God at the point of death. This is the beginning of the Christian teaching of hell below and heaven above.
The Amateur Exegete 
So, as the law requires, I have to point out that I’m not sure it’s accurate to say that Jewish thought collapsed all world history into two ages, one that is evil and one in which God reigns. That seems more like Greco-Roman Christian thought.
I do think he’s right that there is a Greco-Roman shift from a holistic view of what constitutes an “age” or a “reign” to an earthly/spiritual dichotomy where God’s kingdom becomes a purely spiritual construct. The Jewish concept is that the kingdom of God is a concrete political reality. But I don’t see at all that Jewish thought divided all history into two ages, one where God ruled and one where He didn’t. I see much more a succession of ages and the condition of God’s people among the nations potentially differs from age to age.
Even as we move into the post-Exilic hope for the kingdom, there is a dichotomy between the state of being in exile/oppressed and the state of the restored kingdom of God, hence Paul and Jesus can contrast the present age with the age to come, but this does not encompass the entirety of world history, as though the kingdom had never existed prior or world history would basically come to an end with the arrival of the kingdom of God. I think the two-age thing is uniquely Christian and very likely Greco-Roman.
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