The Origin of the Mahdi in Apocalyptic Literature
Now that you have seen the significant dependence that the writers of the Islamic hadiths had on early extra-biblical Christian writings, it should be much easier to convince you that the concept of the Mahdi was derived entirely from the same apocalyptic Christian writings.
In the case of the Islamic Isa and the Dajjal, there is a clear one-to-one comparison with the Christian Jesus and the Antichrist. As we have seen, Isa is based primarily on the Christian Jesus with adjustments for Islamic doctrine. The same is true for the Dajjal where we have seen that the basic concept of the biblical Antichrist was used.
The Islamic Mahdi, on the other hand, is much more interesting in this respect since there is no obvious figure in the Bible that corresponds directly to him. The Bible never mentions a human king who fights religious wars and restores a temporary orthodoxy before the appearance of the Antichrist and the return of Jesus. It would seem at first glance that the writers of the hadith have come up with an entirely new end times character. However, I will attempt to show that it was, in fact, the early Christian writers of the pseudopigrapha that came up with this brand new eschatological character, which was then copied and adapted by the writers of the hadiths to form their concept of the Mahdi.
The new end times character that the Christian extra-biblical apocalyptic writers introduced was a divinely guided monarch who would overcome the present tribulations and usher in a time of temporary peace before the return of Jesus. Though he was not given a name at the time, he would come to be known as the Last Roman Emperor. The primary text that popularized the idea was Pseudo-Methodius, written in the early seventh century, but the Syrian Apocalypse of Daniel played a role as well. The Last Roman Emperor was said to arise at a time when Roman Christianity was in great distress. He would fight a number of wars with the enemies of Christianity and restore Roman Christianity to its previous place of prominence. He would rule for seven to ten years which are described as being particularly plentiful. Then, just before the Antichrist and the Gog-Magog war broke out, Jesus would return, defeat the rebellion, judge the enemies of God, and The Last Roman Emperor would give Jesus his crown.
It is difficult to explain how prominent this idea was at the time. In his paper “The Last Roman Emperor and the Mahdi,” Andras Kraft says that the Last Roman Emperor was given “near-canonical status” at that time and in the centuries that followed. The figure eventually developed into the so-called “Great Monarch,” a concept still believed in certain Catholic circles today. The Last Roman Emperor was also mentioned by Christopher Columbus in his Book of Prophecies written in the early 1500s.
Considering that the concept of the Last Roman Emperor was believed to be true biblical teaching by so many Christians at the time, it is not surprising that Islamic writers incorporated the idea into their eschatology as well. Many of the same early Christian texts from which the hadith writers were borrowing were the same texts that speak of the Last Roman Emperor. In other words, if the Islamic writers were already constructing their doctrines about the Gog-Magog war, the Dajjal, and Isa from Pseudo-Methodius and other similar texts, it is no surprise that they also incorporated the Last Roman Emperor from those same documents into their theology.
The Islamic Antichrist proponents try very hard to find similarities between the Mahdi and the Christian Antichrist. I will argue later that this can only be done in a very general way. But if you choose to compare the Islamic Mahdi with the Last Roman Emperor figure instead of the Antichrist, you can produce a much more impressive list of similarities.
The Mahdi and the Last Roman Emperor share the following characteristics.
They both are human kings.
Both the Mahdi and the Last Roman Emperor are described as purely human, not angelic or divine. Any supernatural things that happen during each of their careers are attributed to God/Allah. The idea in both cases is that God/Allah supports each of these kings and therefore guides and protects them.
They both come at a time of great trouble for their respective religions.
Last Roman Emperor in Pseudo-Methodius :
“Then suddenly tribulation and distress will arise against them. The king of the Greeks, i.e., the Romans, will come out against them in great anger.”
The Last Roman Emperor is preceded by signs very similar to the Islamic “minor signs” which describe a moral decline:
“Men will get themselves up as false women wearing prostitutes’ clothes. Standing in the streets and squares of the cities openly before all they will be adorned like women; they will exchange natural sex for that which is against nature.”
The Mahdi in Islamic tradition:
The Mahdi is frequently mentioned to come on the scene as a result of fitan (trials and tribulations). He is preceded by many of the “minor signs” which describe a time of moral decline. More specifically he is said to come to power to combat the Sufyani , a Muslim tyrant who causes great trouble. The Mahdi is said to defeat the Sufyani once he gains power. 
They both are reluctant to rule.
Last Roman Emperor in Pseudo-Methodius :
“[The Last Roman Emperor will be] roused [in order to rule] as from a drunken stupor like one whom men had thought dead and worthless.”
The Mahdi in Islamic tradition:
“And he [The Mahdi] will accept it [the rule] reluctantly. He will not know, and they will not know, that he is the expected Mahdi, and previously there will be no calls for him to be Mahdi, and he will not even know himself, but God will choose him, and the people will choose him suddenly.” [53 ]
They both fight wars to destroy other human kings opposed to their religious system.
Last Roman Emperor in Pseudo-Methodius :
“He will go forth against them from the Ethiopian sea and will send the sword and desolation into Ethribus (Southern Arabia) their homeland...Egypt will be desolated, Arabia burned with fire, the land of Ausania burned, and the sea provinces pacified. The whole indignation and fury of the king of the Romans will blaze forth against those who deny the Lord Jesus Christ.”
The Mahdi in Islamic tradition:
“Although the uprising of Hadrat al-Mahdi (‘atfs) will commence in Mecca, he will conquer the land of Hijaz” (a large section of Arabia). [54]
“He will have a sword with him, which he will unsheathe, and through him God will conquer the lands of Rome, China, Turkistan, Daylam, Sind, Hind, Kabul, Sham, and Khazar.” [55]
“God will send al-Mahdi (‘atfs) and through him the religion will regain its grandeur and through him and for Him, glorious victories will be attained.” [56]
The primary enemy that he destroys will be a Muslim king who rules over Syria and kills women and children.
In Pseudo-Methodius , the Last Roman Emperor put particular emphasis on destroying Syria :
“The land of Syria will be empty and reduced; those dwelling in her will perish by the sword. . . . Egypt and the East and Syria will be under the yoke and hemmed in by great tribulations. They will be constrained without mercy...The inhabitants of Egypt and Syria will be in trouble and affliction, seven times the greater for those in captivity.”
The reason that the world is in tribulation when he arises is due to the Muslim threat from those regions. The people he destroys are described as “casting lots for children.”
The Mahdi in Islamic tradition:
The Mahdi also places a particular emphasis on destroying Syria when he comes to power. This is because of a threat from “the Sufyani ” an Arabic king who rules of Syria. The Sufyan i also is said to treat women and children badly. 
“A man will emerge from the depths of Damascus [Syria]. He will be called Sufyani . Most of those who follow him will be from the tribe of Kalb. He will kill by ripping the stomachs of women and even kill the children. A man from my family will appear in the Haram, the news of his advent will reach the Sufyani and he will send to him one of his armies. He [referring to the Mahdi] will defeat them. They will then travel with whoever remains until they come to a desert and they will be swallowed. None will be saved except the one who had informed the others about them. (Mustadrak Al-Hakim)”
They both rule only briefly 
Last Roman Emperor in Pseudo-Methodius :
“After this the king of the Romans will go down and live in Jerusalem for seven and half-seven times, i.e., years. When the ten and a half years are completed the Son of Perdition will appear. ”
The Mahdi in Islamic tradition:
The hadiths give different times for the Mahdi’s rule (five, seven, eight, nine or nineteen years), but in any case it is a brief rule. Perhaps one reason for the contradictions in the hadiths is because of the odd way that the length of the rule of the Last Roman Emperor is described in Pseudo-Methodius (i.e. “seven and half-seven times”).
They both succeeded in restoring orthodoxy to their religion, but only briefly.
Last Roman Emperor in Pseudo-Methodius :
The Last Roman Emperor destroys the enemies of Christianity and sets up his religious rule, but this time is followed by terrible destruction, such as the Gog-Magog war and the appearance of the Antichrist. 
The Mahdi in Islamic tradition:
Similarly the Mahdi succeeds in destroying the enemies of Islam and setting up a religious peace, but that peaceful time is followed by the Gog-Magog war and the appearance of the Dajjal, both of those events cause immeasurable destruction to the earth and to Islam.
They both rule over a temporary time of peace and prosperity.
Last Roman Emperor in Pseudo-Methodius :
“The whole indignation and fury of the king of the Romans will blaze forth against those who deny the Lord Jesus Christ. Then the earth will sit in peace and there will be great peace and tranquility upon the earth... ”
The Mahdi in Islamic tradition:
“The Mahdi will appear. Allah will grant him rain, the earth will bring forth its fruits, he will give a lot of money, cattle will increase and the ummah will become great.” [57]
“He will fill out the earth with peace and justice” [58]
Both of their reigns will be followed by the Gog-Magog war.
Last Roman Emperor in Pseudo-Methodius :
The description of the peace the Last Roman Emperor will win for himself is followed by the description of the Gog-Magog war.
“Then the ‘Gates of the North’ will be opened and the strength of those nations which Alexander shut up there will go forth.”
It is clear from the context that follows that the initial peace of the Last Roman Emperor is followed by the Gog-Magog war.
The Mahdi in Islamic tradition:
In Islamic tradition it is clear that the Gog-Magog war takes place after the initial peace of the Mahdi. In addition Isa, not the Mahdi, is ruling at the time of the Gog-Magog war, which conclusively puts the war after the Mahdi’s initial time of peace .
The Antichrist figure comes on the scene at the end of his career.
Last Roman Emperor in Pseudo-Methodius :
“After this the king of the Romans will go down and live in Jerusalem for seven and half-seven times, i.e., years. When the ten and a half years are completed the Son of Perdition will appear .”
The Mahdi in Islamic tradition:
In the hadiths it is quite clear that the Dajjal does not appear until after the Mahdi has defeated Constantinople, an event that occurs toward the end of his career. Isa is said to appear as the Mahdi’s armies, recently returned from Constantinople, are preparing to fight the Dajjal. Isa’s appearance marks the beginning of the end of the Mahdi’s rule.
In both cases Jesus returns at the end of his time after the Antichrist has been revealed.
Last Roman Emperor in Pseudo-Methodius :
Though the actual return of Jesus is not mentioned in Pseudo-Methodius , it does describe the Last Roman Emperor going to Golgotha (the cross of Christ) and laying his crown on the cross (symbolically giving his throne to Jesus). He does this because of the appearance of the Antichrist.
The Mahdi in Islamic tradition:
As mentioned earlier Isa appears after the conquest of Constantinople, when the Mahdi hears of the appearance of the Dajjal. Isa seems to appear for the expressed purpose of defeating the Dajjal, something the Mahdi apparently cannot do.
In both cases he does not defeat the Antichrist.
Last Roman Emperor in Pseudo-Methodius :
As mentioned in the previous point, the Last Roman Emperor, upon hearing of the Antichrist’s appearance, goes to give his throne symbolically to Jesus. He dies at this point. The last words of Pseudo-Methodius make clear that the Antichrist is still on earth after the Last Roman Emperor’s death.
“When the Cross has been lifted up on high to heaven, the king of the Romans will directly give up his spirit. Then every principality and power will be destroyed that the Son of Perdition may be manifest .”
The Mahdi in Islamic tradition:
Here again the Islamic traditions are quite clear that Isa, not the Mahdi, defeats the Dajjal. [59]
“And Allah would then send Jesus son of Mary who would resemble ‘Urwa b Mas'ud.’ He (Jesus Christ) would chase him [the Dajjal] and kill him.” [60]
In both cases Jesus will rule after him .
Last Roman Emperor in Pseudo-Methodius :
This is seen by the Last Roman Emperor abdicating his throne to Jesus and then dying. The text presumes that Jesus will then return to Earth and rule after this. Other material from the time, such as the Apocalypse of Daniel, make this point much more clear.
The Mahdi in Islamic tradition:
It is difficult to know exactly how long the Mahdi lives after Isa arrives. Some Muslims believe he will immediately be killed by a bearded woman or give up the rule of the world to Isa and be killed later. Others believe there will be a short time in which they rule together before Isa takes over. In any case, since Isa is said to rule for forty years after his return and the Mahdi’s rule is said to be much shorter, it is clear that Isa rules after the Mahdi dies.
When you add to this the supplemental information we have already discussed, such as the Gog-Magog war similarities in both versions or the descriptions of the Antichrist/Dajjal having one blind eye and one that shines like a star, it becomes nearly impossible to see these similarities as coincidences. The Mahdi idea, just like so many of the other non-biblical concepts in Islamic eschatology, is based on the peculiar ideas found in the extra-biblical traditions of the early Christian church. It should be noted that, in the case of the Mahdi and the Last Roman Emperor, the writers of the hadiths continued with their method of reversing the religions involved in the original story to make Islam out to be the victor. So in this case, instead of this eschatological hero being a Roman king who fights for Christianity, he is an Islamic king (Caliph) who fights to restore Islam. This is all that is needed to explain the development of the Mahdi idea. That being said, I would recommend the paper I mentioned earlier “The Last Roman Emperor and the Mahdi” by Andras Kraft for more of the background on this subject.