Question: Some people say, "Hadrat 'Isa has started his activities. When he appears soon, he will unite Christians and Muslims against irreligiousness. Thus serenity and peace will prevail on earth. At that time Jews and Christians will follow their own religions, but at the same time they will believe in our Prophet. Therefore, they will be called Musawi or Isawi Muslims. Not everybody will recognize the Mahdi and 'Isa." How will he unite Muslims and Christians? Will he convert Muslims to Christianity?
ANSWER
It is not true in any way and is completely antithetical to our religion. A Qur'anic verse says:
(Certainly, all disbelievers, be they from among Ahl-i Kitab [Jews or Christians] or from among polytheists, will be in the fire of Hell. They will remain there forever. They are the worst of, the most wicked of human beings.) [Al-Bayyina 6; Tafsir al-Qurtubi]

As is seen, the fact that the Ahl al-kitab, that is, Jews and Christians, are kafir (disbelievers) is clearly stated in this verse. Can collaboration ever be established with those about whom Allahu ta'ala says, "They are the worst of human beings" and "They will go to Hell"? Allahu ta'ala tells us explicitly that disbelievers are friends of each other and we must not take them as friends. A Qur'anic verse declares:

(O you who believe! Do not take Jews and Christians as your friends; they are friends of each other [in their animosity toward Islam]. He who takes them as his friends is one of them [a disbeliever]. Allahu ta’ala does not guide the [ones who have become] unjust [to themselves by taking disbelievers as their friends] people.) [Al-Maidah 51]

With the advent of Islam, Christianity and all previous religions, be it true Christianity or an original copy of the Injil and Tawrat, were abrogated and invalidated. If true Christianity or original copies of the Injil and Tawrat were valid, Allahu ta'ala would not have sent Islam and would have said instead, "This is the true religion of 'Isa. Continue to follow this religion." However, he did not say so but said, "The true religion is only Islam" and "I do not accept any religions other than Islam." He rendered Islamic dictates valid until the end of the world.

Even if Christians found the textually uncorrupted Injil and worshiped just as 'Isa 'alaihis-salam prescribed, they would still be in disbelief unless they became Muslims by accepting Muhammad 'alaihis-salam as the true Prophet and Islamic religion as the true religion, for one of the six fundamentals of faith is to believe in all prophets. A person who rejects one of them becomes a disbeliever. Ahl al-kitab are disbelievers. Disbelief is by no means approvable, even though there are those who approve of it.

When 'Isa 'alaihis-salam comes back, he will not mix Zamzam water with wine, but rather he will annihilate Christianity and will spread Islam as a member of the Islamic community. The following hadith-i sharifs declare:
(‘Isa will come as a follower of my religion.) [Imam-i Ahmad]

(‘Isa will descend from the sky as a just judge. He will break the cross [he will wipe out Christianity], kill pigs [he will forbid pork], and prohibit everything other than Islam.) [Bukhari, Muslim, Abu Dawud, Tirmidhi, Ibn Abi Shayba]

(I swear by Allah that ‘Isa will descend among you as a just judge. He will break the cross [he will wipe out Christianity], will kill pigs [he will prohibit eating pork], and will not accept any religions other than Islam.) [Bukhari]

(By Allah, son of Maryam will descend as a just judge, will smash the cross, and will kill pigs. Grudge, hatred, and jealousy will no longer exist then.) [Muslim]

Hadrat ‘Isa, who saw in the original Injil the qualities and superiorities of Muhammad ‘alaihis-salam, begged and prayed very earnestly that he be joined into his Ummah (Muslim community). His prayers were accepted, and Allahu ta’ala raised him up to the heavens alive. Toward the end of the world he will come back down to earth to be a member of Muhammad’s (‘alaihis-salam) Ummah. He will follow his religion and spread it. He will eradicate corrupt religions, such as Christianity and Judaism. (Islam and Christianity)

It is written as follows in the book Kitab al-wafa fi fadail-il- Mustafa:
One day Ka’b al-Ahbar saw a rabbi crying and asked him what reduced him tears. The rabbi did not tell him the reason. Ka’b said, “I will tell you the things that have reduced you to tears, if you wish.” When the rabbi replied in the affirmative, he said: “Having read it in the Tawrat, Musa ‘alaihis-salam said, ‘O my Lord! I saw an Ummah that is the best of umam [pl. of ummah]. In order for people to have faith, they enjoin the doing of what is good and forbid the doing of what is evil. They have belief in the first and the last Books. They will fight Dajjal. Make them my Ummah.’ Allahu ta’ala declared, ‘O Musa! They are Ahmad’s Ummah.’” Listening to these, the rabbi confirmed him, saying, “O Ka’b, you spoke the truth.” Ka’b said, “Having read it in the Tawrat, Musa ‘alaihis-salâm requested, ‘O my Lord! That Ummah praises you much. When they plan to do something, they say insha-Allah. Make them my Ummah.’ Allahu ta’ala declared, ‘O Musa! They are Ahmad’s Ummah.’” The rabbi said, “O Ka’b! You spoke the truth.” Again Ka’b continued to say, “Having read it in the Tawrat, Musa ‘alaihis-salam requested, ‘O my Lord! I see an Ummah that they say takbir when they ascend to a higher place and give thanks when they descend to a lower place. The soil of the earth has been rendered clean for them. With that soil, they become purified from impure substances and ritual impurity just as they become purified from them with water. The earth is their mosque. That is, they do acts of worship wherever they like. Make them my Ummah.’ Allahu ta’ala declared, ‘O Musa! They are Ahmad’s Ummah.’” The rabbi said, “O Ka’b, you spoke the truth.” Thereupon, he continued to relate, “Having read it in the Tawrat, Musa ‘alaihis-salam said, ‘O my Lord! They are such an Ummah that their holy book is in their hearts. When they perform salat, they form lines like angels. From mosques, their voices are heard like honey bees. Few of them will go to Hell. Make them my Ummah.’ Allahu ta’ala declared, ‘O Musa! They are Ahmad’s Ummah.’” The rabbi said, “O Ka’b! You spoke the truth.” When Musa ‘alaihis-salam saw the qualities bestowed upon the Ummah of Muhammad ‘alaihis-salam, he wanted to be a member of his Ummah, and Allahu ta’ala consoled him. (Shawahid-un-nubuwwa)

The greatest ones of the more than 124 thousand prophets that came to the world desired to follow him. If Musa ‘alaihis-salam had lived in his time, despite his greatness, he would have preferred to follow him. Hadrat ’Isa will descend from the heavens and follow his way. Muslims who are of his Ummah became the most auspicious and best of all people because they adapted themselves to him. Most of those who will enter Paradise are from among them, and they will enter before all other people. (Endless Bliss)

When Islam was revealed, it abrogated and nullified all previous religions. A person cannot be with two religions. He/she is either a Muslim or a follower of 'Isa or Musa. To be a Muslim, it is a condition as well to abstain from corrupt and false religions, that is, from disbelief.

It is not correct to claim that not everybody will recognize Hadrat Mahdi and Hadrat 'Isa. The Muslims living at that time will recognize both of them. Three hadith-i sharifs say:

(There will be a cloud just above Mahdi’s head. An angel from the cloud will say, “This is the Mahdi. Listen to his words.”) [Abu Nu’aym]

('Isa will perform salat behind the Mahdi.) [Ibn Hajar-i Makki]

('Isa will be buried next to me.) [Tirmidhi]

(It [Hadrat ‘Isa’s descending from the sky in the time period close to Doomsday] is surely knowledge that is a sign of the approach of Doomsday. Have no doubt about it.) [Az-Zukhruf 61; The Tafsir of Baydawi]

As is seen, it is stated that the coming of Hadrat 'Isa portends the approach of Doomsday and that we must not have doubt about it. This means to say that when he comes back everybody will hear the news of his coming. Despite all these documentary proofs and Hadrat 'Isa's explicitly stated signs, if one still says, "Hadrat Mahdi and Hadrat 'Isa have appeared. Very few people will recognize them, and they will be in collaboration with Christians," be it known that it is contrary to our religion.