Under the premise of belief in Allah and His Messengersa as asserted in the question, the shortest and simplest answer to this would be that it is the will of Allah and His Messengersa that the Promised Messiahas be accepted, and faith in Allah and His Messengersa would be meaningless if one did not comply with their will.
The will of Allah and His Messenger can be derived from the two main sources of Islam: The Quran and the sunnah.
While it is occasionally claimed that the coming of the Promised Messiahas has explicitly been mentioned in the Holy Quran, the fact is that it was only implicitly mentioned there. The explicit prophecy of the coming of the Promised Messiahas is mentioned in the ahadith, the traditions attributed to the Messengersa of Allah, in which it was promised that at a certain time the Messiah would come, hence the Promised Messiah.
Allah says in Holy Quran about the speech of the Messengersa of Allah:
وَمَا يَنطِقُ عَنِ الۡهَوَىٰٓ إِنۡ هُوَ إِلَّا وَحۡيٞ يُوحَىٰ
“And he does not speak out of (his own) desire. It is nothing but (pure) revelation, revealed by God.” (Surah an-Najm, Ch.53: V.4-5)
The Messengersa of Allah said:
أَلاَ إِنِّي أُوتِيتُ الْكِتَابَ وَمِثْلَهُ مَعَهُ أَلاَ يُوشِكُ رَجُلٌ شَبْعَانُ عَلَى أَرِيكَتِهِ يَقُولُ عَلَيْكُمْ بِهَذَا الْقُرْآنِ فَمَا وَجَدْتُمْ فِيهِ مِنْ حَلاَلٍ فَأَحِلُّوهُ وَمَا وَجَدْتُمْ فِيهِ مِنْ حَرَامٍ فَحَرِّمُوهُ أَلاَ لاَ يَحِلُّ لَكُمْ لَحْمُ الْحِمَارِ الأَهْلِيِّ وَلاَ كُلُّ ذِي نَابٍ مِنَ السَّبُعِ وَلاَ لُقَطَةُ مُعَاهِدٍ إِلاَّ أَنْ يَسْتَغْنِيَ عَنْهَا صَاحِبُهَا وَمَنْ نَزَلَ بِقَوْمٍ فَعَلَيْهِمْ أَنْ يَقْرُوهُ فَإِنْ لَمْ يَقْرُوهُ فَلَهُ أَنْ يُعْقِبَهُمْ بِمِثْلِ قِرَاهُ
“Beware! I have certainly been given the Quran and something like it with it. But the time will soon come when a man will sit back on his couch and say: Only follow the Quran, make lawful what you find lawful in it, and forbid what you find unlawful in it.” (Sunan Abi Dawud, Kitab al-Sunnah, Hadith 4604)
This means that if traditions of Allah’s Messengersa reach us that are authentically traceable to him, then it is our duty to accept them. There are ahadith in which he both prophesied the coming of the Promised Messiahas and described the conditions and circumstances of his coming, as well as explained the obligation to accept the Promised Messiahas.
Traditions about the Promised Messiah’s arrival
More than 50 different ahadith transmitted directly from the Messengersa of Allah (marfu), each with a number of narrations, have come down to us about the descent of Jesusas, most of which are classified as authentic (sahih). In addition, there are over 50 ahadith whose chains extend to the Messengersa of Allahprophesying the Promised Messiah under the title of Mahdi (the guided one).
Many eminent scholars of the past and present, such as Imam Ibn Kathirrh (d. 774CE) and Imam al-Shawkanirh (d. 1250CE), have established that the ahadith about the appearance of the Messiah and the Mahdi attain the status of being mass-transmitted (mutawatir) – that they have been transmitted through all generations in such large numbers as to leave no doubt about their authenticity.
The following is a small selection of these ahadith:
وَلاَ تَقُومُ السَّاعَةُ إِلاَّ عَلَى شِرَارِ النَّاسِ وَلاَ الْمَهْدِيُّ إِلاَّ عِيسَى ابْنُ مَرْيَمَ
“[…] and the Hour will only come upon the worst of people, and there will be no Mahdi except Jesus, Son of Mary.” (Sunan Ibn Majah, Kitab al-Fitan, Hadith 4039)
فَإِذَا رَأَيْتُمُوهُ فَبَايِعُوهُ وَلَوْ حَبْوًا عَلَى الثَّلْجِ فَإِنَّهُ خَلِيفَةُ اللَّهِ الْمَهْدِيُّ
“And when you behold him, pledge allegiance to him, even if you have to crawl over the snow, for he is the Khalifa of Allah, the Mahdi.” (Sunan Ibn Majah, Kitab al-Fitan, hadith 4084)
وَالَّذِي نَفْسِي بِيَدِهِ، لَيُوشِكَنَّ أَنْ يَنْزِلَ فِيكُمُ ابْنُ مَرْيَمَ حَكَمًا عَدْلاً، فَيَكْسِرَ الصَّلِيبَ، وَيَقْتُلَ الْخِنْزِيرَ، وَيَضَعَ الْجِزْيَةَ، وَيُفِيضَ الْمَالٓ حَتَّى لاَ يَقْبَلَهُ أَحَدٌ، حَتَّى تَكُونَ السَّجْدَةُ الْوَاحِدَةُ خَيْرًا مِنَ الدُّنْيَا وَمَا فِيهَا
“By Him in Whose hand is my soul, the Son of Mary will soon descend among you as a righteous judge; he will abolish the cross and kill the swine, and there will be no tribute (jizya), and he will distribute wealth such that no one will accept it, and one prostration will be better than the whole world and all that is in it.” (Sahih al-Bukhari, Kitab ahadith al-Anbiya, hadith 3448)
يُوشِكُ مَن عاشَ مِنكُمْ أنْ يَلْقى عِيسى ابْنَ مَرْيَمَ إمامًا مَهْدِيًّا وحَكَمًا عَدْلًا، فَيَكْسِرُ الصَّلِيبَ، ويَقْتُلُ الخِنْزِيرَ، وتُوضَعُ الجِزْيَةَ، وتَضَعُ الحَرْبُ أوْزارَها
“Whoever lives among you shall meet Jesus, Son of Mary, as a rightly-guided (Mahdi) Imam, as a righteous judge. He will break the cross, slay the swine, abolish the tribute (jizya) and the war will lay down its burdens.” (Musnad Ahmad)
لَا تَقُومُ السَّاعَةُ حَتَّى يَنْزِلَ فِيكُمُ ابْنُ مَرْيَمَ حَكَمًا مُقْسِطًا، فَيَكْسِرَ الصَّلِيبَ، وَيَقْتُلَ الْخِنْزِيرَ، وَيَضَعَ الْجِزْيَةَ، وَيَفِيضَ الْمَالُ حَتَّى لاَ يَقْبَلَهُ أَحَدٌ
“The Hour will not be established until the Son of Mary descends amongst you as a just judge, he will break the cross, kill the pigs, and abolish the tribute (jizyah). He will distribute wealth such that nobody will accept it.” (Sahih al-Bukhari, Kitab al-Mazaalim, hadith 2476)
كَيْفَ أَنْتُمْ إِذَا نَزَلَ ابْنُ مَرْيَمَ فِيكُمْ وَإِمَامُكُمْ مِنْكُمْ
“How will you fare when the Son of Mary descends among you and he will be your Imam from among you?” (Sahih al-Bukhari, Kitab ahadith al-Anbiya, hadith 3449)
كَيْفَ أَنْتُمْ إِذَا نَزَلَ فِيكُمُ ابْنُ مَرْيَمَ فَأَمَّكُمْ مِنْكُمْ
“How would you fare when the Son of Mary comes down among you and leads you as one of you?” (Sahih Muslim, Kitab al-Imaan, hadith 155d)
مَنْ أَدْرَكَ مِنْكُمْ عِيسَى ابْنَ مَرْيَمَ فَلْيُقْرِئْهُ مِنِّي السَّلَامَ صَلَّى اللّٰهُ عَلَيْهِمَا وَسَلَّمَ
“Those of you who shall meet Jesus, the Son of Mary, shall convey my greeting of peace (salaam) to him.” (Al-Mustadrak ‘ala Sahihayn)
طُوبَى لِعَيْشٍ بَعْدَ الْمَسِيحِ طُوبَى لِعَيْشٍ بَعْدَ الْمَسِيحِ يُؤْذَنُ لِلسَّمَاءِ فِي الْقَطْرِ وَيُؤْذَنُ لِلأَرْضِ فِي النَّبَاتِ فَلَوْ بَذَرْتَ حَبَّكَ عَلَى الصَّفَا لَنَبَتَ فَلا تَشَاحٍ وَلا تَحَاسُدٌ وَلا تَبَاغُضٌ حَتَّى يَمُرَّ الرَّجُلُ عَلَى الأَسَدِ وَلا يَضُرُّهُ وَيَطَأُ عَلَى الْحَيَّةِ فَلا تَضُرُّهُ وَلا تَشَاحٍ وَلا تَحَاسُدٌ وَلا تَبَاغُضٌ
“Blessed is he who lives after the coming of the Messiah. Blessed is he who lives after the coming of the Messiah. It will be announced for the heavens in the land, and it will be announced for the earth in the crops. If you were to sow your seed upon the rock, it would surely grow. You will not have greed for each other, nor will you envy each other, nor will you hate each other, until a man will pass by a lion and he will not harm him and he will walk over a snake and it will not harm him. You will not have greed for each other, nor will you envy each other, nor will you hate each other.” (Hadith Abi Bakr al-Anbari)
أَرَانِي اللَّيْلَةَ عِنْدَ الْكَعْبَةِ فِي الْمَنَامِ فَإِذَا رَجُلٌ آدَمُ كَأَحْسَنِ مَا يُرَى مِنْ أُدْمِ الرِّجَالِ تَضْرِبُ لِمَّتُهُ بَيْنَ مَنْكِبَيْهِ رَجِلُ الشَّعَرِ يَقْطُرُ رَأْسُهُ مَاءً وَاضِعًا يَدَيْهِ عَلَى مَنْكِبَيْ رَجُلَيْنِ وَهُوَ يَطُوفُ بِالْبَيْتِ فَقُلْتُ مَنْ هَذَا فَقَالُوا هَذَا الْمَسِيحُ ابْنُ مَرْيَمَ ثُمَّ رَأَيْتُ رَجُلًا وَرَاءَهُ جَعْدًا قَطِطًا أَعْوَرَ الْعَيْنِ الْيُمْنَى كَأَشْبَهِ مَنْ رَأَيْتُ بِابْنِ قَطَنٍ وَاضِعًا يَدَيْهِ عَلَى مَنْكِبَيْ رَجُلٍ يَطُوفُ بِالْبَيْتِ فَقُلْتُ مَنْ هَذَا قَالُوا الْمَسِيحُ الدَّجَّالُ
“I saw a vision last night by the Ka‘bah in my sleep. There was a brown-skinned man, as beautiful as I had ever seen. His hair fell between his shoulders, a man whose head was dripping with water. He placed his hands on the shoulders of two men as he circled the House. I said: Who is this? They said: It is the Messiah, Son of Mary. Then I saw a man behind him, with very curly hair and blindness in his right eye, resembling the look of Ibn Qatan. He placed his hands on the shoulders of a man as he circled the House. I said: Who is this? They said: It is the False Messiah.” (Sahih al-Bukhari, Kitab ahadith al-Anbiya, hadith 3439)
يَنْزِلُ عِيسَى ابْنُ مَرْيَمَ صَلّٰى اللّٰهُ عَلَيْهِ وَسَلَّمَ فَيَقُولُ أَمِيرُهُمْ تَعَالَ صَلِّ لَنَا فَيَقُولُ لَا إِنَّ بَعْضَكُمْ عَلَى بَعْضٍ أُمَرَاءُ تَكْرِمَةَ اللّٰهِ هَذِهِ الْأُمَّةَ
“Jesus, the Son of Mary will descend and the Muslim leader will say: Come lead us in prayer. Jesus will say: No, some of you are leaders over others, as Allah has honoured this Ummah.” (Sahih Muslim, Kitab al-Imaan, hadith 156)
Furthermore, there are countless prophecies by saints and sages (awliya) of Islam prophesying the coming of the Messiah and the Mahdi, as well as statements by Islamic scholars and jurists stating that it is incumbent upon Muslims to follow the Messiah and Mahdi once he appears.
(Prepared by the Ahmadiyya Archive and Research Centre)