During the time of the Imperial Church, changes in eschatological views began to shift, specifically on the millennium. The Alexandrian allegorical interpretation espoused by Origen and others had a lasting impact in the area of eschatology. Tyconius wrote Book of Rules where he presented his interpretive method for the scriptures. He taught that the prophecies of scripture will be fulfilled spiritually, not literally as the Early Church had imagined. According to his interpretation of Revelation 20:1-6, the one thousand years is a spiritual reign with Christ and corresponds to the current church age. Thus, due to the rejection of a literal millennium, Tyconius’ view has been called “amillennialism.”
Tyconius influenced Augustine who further explained the amillennial view by linking the bondage of Satan for a thousand years in Revelation 20:2-3 with Jesus’ casting out demons as a result Satan being bound (Mark 3:27) and by referring to Luke 10:18 where Jesus states, “I saw Satan fall like lightning from heaven.” According to Augustine, when the church began to extend throughout the nations, this revealed that Satan’s power had been restrained and crippled. It is only when Satan is subdued that people experience the “first resurrection” spiritually, and then reign with Christ spiritually until he comes a second time at the end of the world. While Augustine taught that there was no literal future millennium, he did posit that there was a future eternal millennium that symbolically rounded out “the seven millenniums from Adam which he held comprised the history of man.”
On a more superficial level, Augustine may have avoided the literal interpretation of the millennium due to a popular view that the millennium emphasized “luxurious material blessings” or that it was “a time of carnal enjoyment.” This view may have formed within the context of the new changes between church and state under Constantine’s prioritizing of Christianity. Regardless of Augustine’s exact reasoning for his rejection of the literal millennium, his “amillennial interpretation of Revelation 20 superseded the premillennial understanding so prevalent in the early church. With this development, amillennialism became the dominant eschatological belief for well over the next millennium.”
Beyond his amillennial views, Augustine developed his eschatology into a philosophy of history in his treatise entitled City of God. In this work, he describes two cities—the city of God and the city of the world. These two cities battle throughout the ages, but God and His city prevail in the end. The story of these two cities presents a strong Christian eschatology where the church is exiled in the present secular world during the time between Christ’s incarnation and his glorious return. There is an eschatological tension experienced by those of the city of God as they live out the Christian faith surrounded by unbelief while at the same time looking to the future hope where they will finally be delivered and be with God in his glory for eternity.
In the City of God, Augustine presents his views on final judgement and eternal punishment. He followed the NT writers and the Early Church Fathers on these views in the face of several opponents. There was the continued influence of Origen’s allegorical theology that posited that God would ultimately restore and save every fallen being, and so punishment was viewed as rehabilitative and not eternal. Some argued that eternal punishment was impossible seeing that continued punishment would eventually destroy embodied human beings. Others suggested that there were no sins that one could commit which would warrant eternal punishment or that God, due to his compassionate character, would eventually cease punishment. In response to these arguments, Augustine, referring to Matthew 25:46, wrote,
If both destinies are “eternal,” then we must either understand both as long continued but at last terminating, or both endless. For they are correlated: On the one hand, punishment eternal; on the other hand, life eternal. And to say in one and the same sense ”life eternal will be endless, punishment eternal will come to an end,” is the height of absurdity. Therefore, as the eternal life of the saints will be endless, so too the eternal punishment of those who are doomed to it will have no end.
 Allison, Historical Theology, EPUB edition, pt.7, ch. 31, “Eschatological Beliefs in the Early Church.”  Allison, Historical Theology, EPUB edition, pt.7, ch. 31, “Eschatological Beliefs in the Early Church.”  John F. Walvoord, “Amillennialism From Augustine to Modern Times” Bibliotheca Sacra 106, no. 424 (1949), 423.  Allison, Historical Theology, EPUB edition, pt.7, ch. 31, “Eschatological Beliefs in the Early Church.”  Walvoord, “Amillennialism From Augustine to Modern Times,” 423.  Allison, Historical Theology, EPUB edition, pt.7, ch. 31, “Eschatological Beliefs in the Early Church.”  Allison, Historical Theology, EPUB edition, pt.7, ch. 31, “Eschatological Beliefs in the Early Church.”  Noble, “Eschatology,” in NDT, EPUB edition.  McGrath, Christian Theology, EPUB edition, pt.3, ch. 18, “Augustine: the two cities.”  Allison, Historical Theology, EPUB edition, pt.7, ch. 31, “Eschatological Beliefs in the Early Church.”  Augustine, City of God, 21.3, in NPNF, 2:453-54 quoted by Allison, Historical Theology, EPUB edition, pt.7, ch. 31, “Eschatological Beliefs in the Early Church.”