During the Medieval Period, eschatology began to take on a new emphasis from that of the earlier church ages. Islam was expanding and had conquered Jerusalem which led the church to believe that Islam was the antichrist of the end times. Moreover, natural disasters, famines and the bubonic plague throughout Europe were viewed as precursors to the end of the world.
One of the ways that the church responded to these circumstances is by organizing military crusades. An example of the eschatological thought behind the crusades is seen in the views of Bernard of Clairvaux who led and inspired the Second Crusade.
Bernard developed a four-age theory for Christian eschatology which was linked to the four trials in Psalms 91:5-6, “You will not fear the terror of the night, nor the arrow that flies by day, nor the pestilence that stalks in darkness, nor the destruction that wastes at noonday.” These four trials described the four time periods of the age of martyrs, the age of heretics, the age of corruption, and the age of the antichrist. With Bernard’s theory many believed that they were either in the latter stages of the age of corruption or in the beginning stages of the age of the antichrist. Regardless of which age people viewed as the present, they believed that participating in the crusades would contribute to the fulfillment of eschatological prophecies and the second coming of Christ. This anticipation of the last days caused Bernard to lead the Second Crusade which was going against everything he had stood for as a Cistercian monk.
Another popular eschatological view during the Medieval Period was developed by Joachim of Fiore. According to Joachim, history was based on three ages in connection with the doctrine of the Trinity. Thus, the age of the Father, corresponding with the period of the Old Testament; the age of the Son, corresponding with the Jesus movement and the church; the age of Spirit, corresponding to a renewal of the church and the final state of unity and peace on earth. Joachim argued that each age “consisted of 42 generations of 30 years each. As a result, the age of the Son was due to end in 1260 to be followed immediately by the radical new ‘age of the spirit’.” According to Joachim, the end of second age was drawing near resulting in an antichrist led tribulation against the church. In order to guide the church through this time, Joachim believed that God would establish two new orders of monks corresponding to the two witnesses of Revelation 11:3-12. One order was described as the preachers and another as the hermits and despite the witness of the preachers and the prayers of the hermits, the antichrist would prevail over the church and turn it towards evil. However, the return of Christ would end this dreadful state ushering in the third age defined by spiritual renewal, peace and blessing. Joachim’s views challenged the amillennial views of the medieval church, but his views were condemned by the Fourth Lateran Council in 1215.
The eschatological views of Thomas Aquinas were the most influential during the Medieval Period . Aquinas upheld Augustine’s amillennialism and the historic doctrines of the return of Christ, the last judgment, eternal punishment and the new heaven and new earth. A significant amount of Aquinas’ writings concerning eschatology was focused on responding to those who denied eternal punishment of the wicked. Aquinas pointed to Jesus’ teaching about eternal punishment in Matthew 25:46 which states, “And these [the unrighteous] will go away into eternal punishment, but the righteous into eternal life.” Aquinas expounded on eternal punishment by referring to the “pain of loss” and the “pain of sense.” In the “pain of loss”, those who are against God lose the “beatific vision of God” and are “punished by their exclusion from perpetual happiness.” In the “pain of sense,” the wicked experience the sense of painful torment of body and the soul.
As far as his views on the doctrine of the new heavens and new earth, Aquinas emphasised the renewal of the heavens and earth rather than annihilation before the creation of the new. He cited Isaiah 65:17 which states, “Behold I create new heavens and a new earth, and the former things shall not be in remembrance” and Revelation 21:1 which states, “I saw a new heaven and a new earth. For the first heaven and the first earth was gone.” Moreover, he reasoned that “the dwelling should befit the dweller. But the world was made to be man’s dwelling. Therefore, it should befit man. Now man will be renewed. Therefore, the world will be likewise.”
Lastly, a discussion on medieval eschatology would be incomplete without a mention of the idea of purgatory, which was an ancient idea that was further developed with new emphasis during the Medieval Period. Thus, the medieval idea posited that “the ‘eternal punishment’ due to sin is waived, but there remains a ‘temporal punishment’ to pay.” When people die without having paid off the temporal punishment debt, they enter purgatory where the debt is paid off by “penal and expiatory suffering.” This suffering was believed to be lessened through the offering of mass for the departed, and the offering of indulgences was viewed as releasing people from purgatory.
The medieval period was a time of shifting eschatological thought for the church due to external political, military and natural challenges. The church responded to these challenges with an apocalyptic zeal or with a scholastic biblical outlook. During this period, purgatory became the central eschatological focus which was one of the many factors leading to the Protestant Reformation.
 Allison, Historical Theology, EPUB edition, pt.7, ch. 31, “Eschatological Beliefs in the Middle Ages.”  Maria L. Ruby Wagner, “The Impact of the Second Crusade on the Angelology and Eschatology of Saint Bernard of Clairvaux,” Journal of Religious History 37, no. 3 (2013), 326.  Wagner, “The Impact of the Second Crusade on the Angelology and Eschatology of Saint Bernard of Clairvaux,” 328.  McGrath, Christian Theology, EPUB edition, pt.3, ch. 18, “Joachim of Fiore: three ages.”  McGrath, Christian Theology, EPUB edition, pt.3, ch. 18, “Joachim of Fiore: three ages.”  These two new orders of monks were identified as the Franciscan order and the Dominican order. For an example of how Joachim’s views influenced these orders of monks, see Ray C. Petry “Medieval Eschatology and St. Francis of Assisi.” Church History 9, no. 1 (1940): 54-69.  Allison, Historical Theology, EPUB edition, pt.7, ch. 31, “Eschatological Beliefs in the Middle Ages.”  McGrath, Christian Theology, EPUB edition, pt.3, ch. 18, “Joachim of Fiore: three ages.”  Allison, Historical Theology, EPUB edition, pt.7, ch. 31, “Eschatological Beliefs in the Middle Ages.”  Thomas Aquinas, Summa Theologica, pt. 3, sup., q. 99, art. 1 http://www.ccel.org/ccel/ aquinas/summa.XP_Q99_A1.html.  Allison, Historical Theology, EPUB edition, pt.7, ch. 32, “Judgment and Punishment in the Middle Ages.”  Allison, Historical Theology, EPUB edition, pt.7, ch. 32, “Judgment and Punishment in the Middle Ages.”  Thomas Aquinas, On the Truth of the Catholic Faith, 218, quoted in Robert A. Peterson, Hell on Trial: The Case for Eternal Punishment (Phillipsburg, N.J. : P&R, 1995), 109, cited by Allison, Historical Theology, EPUB edition, pt.7, ch. 32, “Judgment and Punishment in the Middle Ages.”  Allison, Historical Theology, EPUB edition, pt.7, ch. 32, “Judgment and Punishment in the Middle Ages.”  Aquinas, Summa, pt. 3, sup., q. 91, art. 1 http://www.ccel.org/ccel/aquinas/summa.XP_Q91 _A1.html.  Aquinas, Summa, pt. 3, sup., q. 91, art. 1 http://www.ccel.org/ccel/aquinas/summa.XP_Q91 _A1.html.  A.N.S Lane, “Purgatory” in New Dictionary of Theology (NDT) : Historical and Systematic edited by Martin Davie et al. 2nd ed, (Downers Grove, Illinois: IVP Academic, 2016), EPUB edition.  Lane, “Purgatory” in NDT, EPUB edition. Lane, “Purgatory,” in NDT, EPUB edition.  Lane, “Purgatory,” in NDT, EPUB edition.