If you really want to understand ISIS, you must seek to understand their apocalyptic doctrine, or their Islamic eschatology. If you listen carefully to their rantings you might pick up some of it. Hence, they are supremely motivated by their eschatology and by the fact that they believe their messiah, the Mahdi, is coming soon.
The following is an excerpt of a blog by Joel Rosenthal. It lays out clearly the apocalyptic doctrine of ISIS.
Indeed, for the first time in human history, the top leaders of not just one nation state but two — Iran and the Islamic State — are being driven by Islamic eschatology, or End Times theology. Their particular brands of Shia and Sunni eschatology are driving them towards genocide. Why? Because they believe:
that the End of Days have arrived
that the Islamic messiah known as the “Mahdi” will appear at any moment [read more on the Mahdi in an article by Michael Youssef below].
that when the Mahdi appears, he will rule the entire Earth
that Jesus will also return to Earth, but not as the Messiah, or Savior, or Son of God, but as the deputy to the Mahdi
that Jesus will force all Jews, Christians and other so-called “infidels” to convert to Islam or be executed
that the way to hasten the arrival and full establishment of the global Islamic kingdom or “caliphate” is to annihilate Jews and Christians, and specifically to annihilate Israel (which they call the “Little Satan” in their eschatology), and the United States (which they call the “Great Satan.”)
that time is very short, and they must move decisively because soon each Muslim will face the Mahdi face to face and be brought into judgment if they have not faithfully followed the Mahdi’s orders.
The Antichrist and the Muslim Mahdi (Part 2) by Michael Youssef
Because Islam borrowed a great deal from Judaism and Christianity, it is not surprising that they too have a figure with a messianic complex—the Mahdi, or the Guided One.
Muslims see the Mahdi as a savior who will lead a global revolution and establish a worldwide Islamic empire. The Mahdi will rule the earth as the final Caliph of Islam (a caliph is both a political ruler and a spiritual representative of Allah on earth).
Muslims revere Jesus (whom they call Isa) as the Masih or Messiah. In Islamic eschatology, the Mahdi will arrive at the same time that Isa returns. Isa will descend to earth in Syria, east of Damascus, dressed in yellow robes, and will assist the Mahdi, who will rule over the earth for seven years (or, in some Islamic traditions, nine or nineteen years). At the end of the Mahdi’s rule, there will be a Day of Judgment for the entire human race.
The parallels between the biblical Antichrist and the Mahdi of Islam are chilling. Both are associated with the End Times and the Judgment. Both possess political, military, and religious power, and head up a one-world religion. The Mahdi will force all non-Muslim people to convert to Islam. Like the Antichrist, the Mahdi will establish Jerusalem as his capital, from which he will rule the earth. (click here to read the entire article)