Allāh said:

شَرَعَ لَكُم مِّنَ الدِّينِ مَا وَصَّىٰ بِهِ نُوحًا وَالَّذِي أَوْحَيْنَا إِلَيْكَ وَمَا وَصَّيْنَا بِهِ إِبْرَاهِيمَ وَمُوسَىٰ وَعِيسَىٰ ۖ أَنْ أَقِيمُوا الدِّينَ وَلَا تَتَفَرَّقُوا فِيهِ ۚ كَبُرَ عَلَى الْمُشْرِكِينَ مَا تَدْعُوهُمْ إِلَيْهِ ۚ اللَّهُ يَجْتَبِي إِلَيْهِ مَن يَشَاءُ وَيَهْدِي إِلَيْهِ مَن يُنِيبُ ‎

“Allāh has ordained for you the same religion [of Islām] which He ordained for Nūḥ, and that which We have inspired in you [O’ Prophet (صلى الله عليه وسلم)], and that which We ordained for Ibrāhīm, Mūsá and ʿĪsá saying you should establish the religion and not be divided therein. Intolerable for the pagans is that to which you [O’ Prophet (صلى الله عليه وسلم)] call them. Allāh chooses for Himself whom He wills, and guides unto Himself who turns to Him in repentance and in obedience.”
[Al-Shūrá, 42:13]

Allāh, the Exalted, addresses this ummah saying, “Allāh has ordained for you the same religion [of Islām] which He ordained for Nūḥ, and that which We have inspired in you [O’ Prophet (صلى الله عليه وسلم)].” Here, Allāh mentions the first messenger sent to mankind after [the creation of] Ādam, Nūḥ, and the last of them, Muḥammad (صلى الله عليه وسلم). Thereafter, He mentioned those with firm will and resolution among the Messengers, and they are Ibrāhīm, Mūsá and ʿIsá the son of Maryam. This verse mentions these five, just as the verse of the chapter of al-Aḥzāb mentions them, and that is Allāh’s saying:

وَإِذْ أَخَذْنَا مِنَ النَّبِيِّينَ مِيثَاقَهُمْ وَمِنكَ وَمِن نُّوحٍ وَإِبْرَاهِيمَ وَمُوسَىٰ وَعِيسَى ابْنِ مَرْيَمَ ۖ وَأَخَذْنَا مِنْهُم مِّيثَاقًا غَلِيظًا ‎‏

“And [remember] when We took from the Prophets their covenant, and from you [O’ Prophet], and from Nūḥ, Ibrāhīm, Mūsá, and ʿIsá the son of Maryam. We took from them a strong covenant.”
[Al-Aḥzāb, 33:7]

The religion which all the messengers called the people to is the worship of Allāh alone, without any partners, as Allāh said:

وَمَا أَرْسَلْنَا مِن قَبْلِكَ مِن رَّسُولٍ إِلَّا نُوحِي إِلَيْهِ أَنَّهُ لَا إِلَٰهَ إِلَّا أَنَا فَاعْبُدُونِ ‏

“We did not send any messenger before you, [O’ Prophet], except that We inspired him [saying], ‘None has the right to be worshipped but I (i.e., Allāh), so worship Me [alone].”
[Al-Anbiyāʾ, 21:25]

There also occurs in the ḥadīth that the Prophet (صلى الله عليه وسلم) said, “The prophets are paternal brothers, our religion is one.”^[1] Meaning, that which is shared amongst all the prophets is [the command to] worship Allāh alone, without any partners. Despite that however, it is possible for their legislations and ways to be different from one another, as Allāh says:

لِكُلٍّ جَعَلْنَا مِنكُمْ شِرْعَةً وَمِنْهَاجًا

“To each (nation) among you (from the nations of Mūsá, ʿĪsá and Muḥammad), We prescribed a law and a (clear) way.”
[Al-Māʾidah, 5:48]

Hence, Allāh says in this verse [of al-Shūrá], “…Saying you should establish the religion and not be divided therein.” Therefore, Allāh ordained for all the prophets that they be united, and He forbade them from splitting and differing. Thereafter, He said, “Intolerable for the pagans is that to which you [O’ Prophet (صلى الله عليه وسلم)] call them.” That is, they find intolerable and utterly objectionable, what you call them to of tawḥīd.^[2]