In ayat 3:81, Al-Quran mentions a covenant of the prophets with Allah. We will analyze related ayahs in this post.
3:81 وَإِذْ أَخَذَ اللَّهُ مِيثَاقَ النَّبِيِّينَ لَمَا آتَيْتُكُم مِّن كِتَابٍ وَحِكْمَةٍ ثُمَّ جَاءَكُمْ رَسُولٌ مُّصَدِّقٌ لِّمَا مَعَكُمْ لَتُؤْمِنُنَّ بِهِ وَلَتَنصُرُنَّهُ ۚ قَالَ أَأَقْرَرْتُمْ وَأَخَذْتُمْ عَلَىٰ ذَٰلِكُمْ إِصْرِي ۖ قَالُوا أَقْرَرْنَا ۚ قَالَ فَاشْهَدُوا وَأَنَا مَعَكُم مِّنَ الشَّاهِدِينَ
And when Allah took the covenant from the prophets, saying. "That I have given you from book and wisdom; then comes to you a messenger, confirming what is with you; then you do believe in him and render him help." Allah said. "Do you render your consent, and take this my burden on you?" They said. "We agree." He said. "Then bear witness, and I am with you among the witnesses."
3:82 فَمَن تَوَلَّىٰ بَعْدَ ذَٰلِكَ فَأُولَٰئِكَ هُمُ الْفَاسِقُونَ
If any turn back after this, they are perverted transgressors.
A covenant of Allah and His prophets is mentioned in this ayat, that when they get from the Book and Wisdom, then comes to them a messenger; they should then indeed believe in him and support him. A school of scholars have the view that the messenger mentioned in this ayat is 'Mohammad' (pbuh). However the problem with that view point is that the prophets were sent at different time periods and Mohammad (pbuh) as a 'bashar' and human cannot travel in time to take covenant from other prophets. In addition this covenant is also taken from the messenger Mohammad (pbuh) as per below ayat 33:7.
33:7 وَإِذْ أَخَذْنَا مِنَ النَّبِيِّينَ مِيثَاقَهُمْ وَمِنكَ وَمِن نُّوحٍ وَإِبْرَاهِيمَ وَمُوسَىٰ وَعِيسَى ابْنِ مَرْيَمَ ۖ وَأَخَذْنَا مِنْهُم مِّيثَاقًا غَلِيظًا
And when We took from the prophets their covenant and from you (the messenger pbuh) and from Noah, Abrahim, Mosa, and Essa the son of Mary and We took from them a solemn covenant.
At a few places in Quran, Allah specifically mentioned that the messenger Mohammad pbuh, was not present in the past eras, such as with prophet Moosa (a.s) when Allah called him from the side of Toor. Therefore prophet Mohammad pbuh was not the one present in his era and could not take covenant from other prophets.
28:46 وَمَا كُنتَ بِجَانِبِ الطُّورِ إِذْ نَادَيْنَا وَلَٰكِن رَّحْمَةً مِّن رَّبِّكَ لِتُنذِرَ قَوْمًا مَّا أَتَاهُم مِّن نَّذِيرٍ مِّن قَبْلِكَ لَعَلَّهُمْ يَتَذَكَّرُونَ
You (the messenger pbuh) were not at the side of Tur when We called. Yet a mercy from your Lord, so that you give warning to a people to whom no warner had come before you so that they may remember.
Some scholars hold the view that this covenant took place in the realm of sprit world, when the prophets were not sent to the world yet. But then if that had taken place at that time, the prophet (pbuh) would have known about this covenant and he would have been to previous prophets to take covenant, but the ayat 11:49 says that you (the messenger pbuh) did not know about the the events of previous prophets.
11:49 تِلْكَ مِنْ أَنبَاءِ الْغَيْبِ نُوحِيهَا إِلَيْكَ ۖ مَا كُنتَ تَعْلَمُهَا أَنتَ وَلَا قَوْمُكَ مِن قَبْلِ هَٰذَا ۖ فَاصْبِرْ ۖ إِنَّ الْعَاقِبَةَ لِلْمُتَّقِينَ
Such are some of the stories of the unseen, which We have revealed unto thee, before this, neither thou nor your people knew them. So persevere patiently. for the End is for those who are righteous.
Another school of thought is that the covenant was for prophets including their nations. So when the messenger Mohammad (pbuh) would come to the nations of previous prophets then they should believe in him and support him. However the problem with this view point is that the ayat specifically mentions the covenant of the prophets and does not mention their nations. So, we cannot assume the covenant of prophets to include their nations as well.
It is a no brainer that for a human/bashar to be present in all prophets times and taking covenant from them is not possible. Therefore this messenger mentioned in ayat 3:81 has to be either an angel or Rooh, the Sprit. The angels and Rooh are stated to be messenger in Al-Quran by Allah.
16:2 يُنَزِّلُ الْمَلَائِكَةَ بِالرُّوحِ مِنْ أَمْرِهِ عَلَىٰ مَن يَشَاءُ مِنْ عِبَادِهِ أَنْ أَنذِرُوا أَنَّهُ لَا إِلَٰهَ إِلَّا أَنَا فَاتَّقُونِ
He sends down His angels with sprit for His Command, to such of His servants as He pleases, (saying). "Warn (Man) that there is no god but I. so do your duty unto Me."
The Rooh/Sprit is the spirit of the Words of Quran. Allah mentions sending the Spirit to Mariam a.s which was in the form of human.
19:17 فَاتَّخَذَتْ مِن دُونِهِمْ حِجَابًا فَأَرْسَلْنَا إِلَيْهَا رُوحَنَا فَتَمَثَّلَ لَهَا بَشَرًا سَوِيًّا
Then she took a hijab/veil apart from them, then We sent to her Our Rooh/Spirit. So he was an even example of a human for her.
Before the spirit comes, the prophets don't have the true understanding of the message of Allah.
42:52 وَكَذَٰلِكَ أَوْحَيْنَا إِلَيْكَ رُوحًا مِّنْ أَمْرِنَا ۚ مَا كُنتَ تَدْرِي مَا الْكِتَابُ وَلَا الْإِيمَانُ وَلَٰكِن جَعَلْنَاهُ نُورًا نَّهْدِي بِهِ مَن نَّشَاءُ مِنْ عِبَادِنَا ۚ وَإِنَّكَ لَتَهْدِي إِلَىٰ صِرَاطٍ مُّسْتَقِيمٍ
And Likewise, We have inspired you the Rooh/Spirit from our order, you did not have the perception, what is The Book and the Belief, but We have made it light, We guide with it whom We will from Our servants. And surely you guide towards the straight path.
When the Spirit becomes apparent to the messengers then they are obliged to support it. How to support the spirit, when it comes to them is to respond to it and start spreading the message of Allah to the masses.
5:67 يَا أَيُّهَا الرَّسُولُ بَلِّغْ مَا أُنزِلَ إِلَيْكَ مِن رَّبِّكَ ۖ وَإِن لَّمْ تَفْعَلْ فَمَا بَلَّغْتَ رِسَالَتَهُ ۚ وَاللَّهُ يَعْصِمُكَ مِنَ النَّاسِ ۗ إِنَّ اللَّهَ لَا يَهْدِي الْقَوْمَ الْكَافِرِينَ
O' Messenger ! proclaim The (message) which has been sent to you from your Lord. If you did not, you would not have proclaimed His message. And Allah will defend you from masses, for Allah guides not the nation of rejecters.
Al-Quran mentions aiding prophet Essa ibne Mariam with Rooh.
2:87 وَلَقَدْ آتَيْنَا مُوسَى الْكِتَابَ وَقَفَّيْنَا مِن بَعْدِهِ بِالرُّسُلِ ۖ وَآتَيْنَا عِيسَى ابْنَ مَرْيَمَ الْبَيِّنَاتِ وَأَيَّدْنَاهُ بِرُوحِ الْقُدُسِ ۗ أَفَكُلَّمَا جَاءَكُمْ رَسُولٌ بِمَا لَا تَهْوَىٰ أَنفُسُكُمُ اسْتَكْبَرْتُمْ فَفَرِيقًا كَذَّبْتُمْ وَفَرِيقًا تَقْتُلُونَ
We gave Moosa the Book and followed him up with a succession of messengers; We gave Essa the son of Mary Clear Signs and strengthened him with the holy spirit. Is it that whenever there comes to you a messenger with what your nafs/soul desire not, you are puffed up with pride?- Some of them you called impostors, and others you slew!
The Spirit is the Messenger to all mankind, past and present and proclaims as such in the below ayat and asks them to believe in the Ummi prophet.
7:158 قُلْ يَا أَيُّهَا النَّاسُ إِنِّي رَسُولُ اللَّهِ إِلَيْكُمْ جَمِيعًا الَّذِي لَهُ مُلْكُ السَّمَاوَاتِ وَالْأَرْضِ ۖ لَا إِلَٰهَ إِلَّا هُوَ يُحْيِي وَيُمِيتُ ۖ فَآمِنُوا بِاللَّهِ وَرَسُولِهِ النَّبِيِّ الْأُمِّيِّ الَّذِي يُؤْمِنُ بِاللَّهِ وَكَلِمَاتِهِ وَاتَّبِعُوهُ لَعَلَّكُمْ تَهْتَدُونَ
Say. "O Mankind! I am the messenger of Allah,to you all to; to whom belongs the kingdom of the heavens and the earth. There is no god but He. It is He who gives life and death. So believe in Allah and His Messenger, the Ummi Prophet, who believes in Allah and His words. And follow him so that you may be guided."