Allāh (the Most High) has commanded us with unity upon the truth as He stated:

وَاعْتَصِمُوا بِحَبْلِ اللَّهِ جَمِيعًا وَلَا تَفَرَّقُواۚ

“And hold fast to the Rope of Allāh, altogether and be not divided.” (Āli ʿImrān: 103) He also said:

إِنَّ الَّذِينَ فَرَّقُوا دِينَهُمْ وَكَانُوا شِيَعًا لَّسْتَ مِنْهُمْ فِي شَيْءٍ ۚ إِنَّمَا أَمْرُهُمْ إِلَى اللَّهِ ثُمَّ يُنَبِّئُهُم بِمَا كَانُوا يَفْعَلُونَ

“Indeed, those who have divided their religion and become sects, you O Prophet, are not associated with them in the least. Their affair is left only to Allāh and He will inform them about what they used to do.” (Al-Anʿām: 159)

Regarding those who divide and become sects, Al-Baghawī (Allah’s mercy be upon him) said, “They are the people of innovation and desires.” Ibn Al-Mubārak (Allah’s mercy be upon him) said, “The people of truth have no differing among them.”. Ash-Shātibi (Allah’s mercy be upon him) stated, “Splitting is from the particular characteristics of the innovators.” 

[Al-Baghawī: (Sharh As-Sunnah, 1/210). Ibn Al-Mubārak: (At-Tabarī in Jāmi’ Al-Bayān, 12/85). Ash-Shātibi: (Al-I’tisām, 1/113)]

And He, the Most High, also stated:

وَلَا تَكُونُوا كَالَّذِينَ تَفَرَّقُوا وَاخْتَلَفُوا مِن بَعْدِ مَا جَاءَهُمُ الْبَيِّنَاتُ ۚ وَأُولَٰئِكَ لَهُمْ عَذَابٌ عَظِيمٌ

“And do not be like the ones who became divided and differed after the clear proofs had come to them. And those will have a great punishment.” (Āli ʿImrān: 105) So Allāh prohibited us from division and commanded us with unity and holding fast to the Book of Allāh and the Sunnah of His Messenger (peace and blessings be upon him), as He said:

وَأَنَّ هَٰذَا صِرَاطِي مُسْتَقِيمًا فَاتَّبِعُوهُ ۖ وَلَا تَتَّبِعُوا السُّبُلَ فَتَفَرَّقَ بِكُمْ عَن سَبِيلِهِ ۚ ذَٰلِكُمْ وَصَّاكُم بِهِ لَعَلَّكُمْ تَتَّقُونَ

“And this is My Straight Path, so follow it; and do not follow the other paths, for you will be separated from His Path. This He has instructed you with, that you may become righteous.” (Al-Anʿām: 153)

So, differing, splitting [by following one’s desires], blindly-following one’s father and forefathers, and blindly-following the Jews and Christians are all impermissible.

Differing is not permissible in the affairs of belief and in the foundations of the religion; it is obligatory to agree and unite upon these matters. As for differing that takes place in matters of fiqh, then this can occur, but even here one must only accept sayings that are supported by the proofs. Allāh (the Most High) stated:

فَإِن تَنَازَعْتُمْ فِي شَيْءٍ فَرُدُّوهُ إِلَى اللَّهِ وَالرَّسُولِ إِن كُنتُمْ تُؤْمِنُونَ بِاللَّهِ وَالْيَوْمِ الْآخِرِ ۚ ذَٰلِكَ خَيْرٌ وَأَحْسَنُ تَأْوِيلًا

“And if you disagree over anything, refer it to Allāh and the Messenger, if you believe in Allāh and the Last Day. That is the best way and best in result.” (An-Nisā’: 59) The point is that differing in matters pertaining to belief (i.e. the ʿaqīdah) is not permitted because the ʿaqīdah is established by textual proofs alone and not open to juristic interpretation or opinion. As for the fiqh rulings and derived judgements, those who are capable and worthy from the scholars will deduce what they see to be most correct from the sources of the religion.

This is the way of Ahlus-Sunnah wal-Jamāʿah and this is what the Messenger (peace and blessings be upon him) commanded with. As for the saying of the ignorant, “Leave the people alone, each to his own opinion because differing in the ummah is a mercy.” We say, this is sheer falsehood. Allāh (the Most High) stated:

وَلَا يَزَالُونَ مُخْتَلِفِينَ إِلَّا مَن رَّحِمَ رَبُّك

“But they will not cease to differ except whom your Lord has shown mercy.” (Hūd: 118-119) The saying of Allāh (the Most High), “Except whom your Lord has shown mercy” is a proof that those who are shown mercy by Allāh do not differ, and that differing is a punishment and not a mercy.

Ibn Mas’ūd (Allah be pleased with him) said, “Differing is evil.” At-Tahāwi (Allah’s mercy be upon him) said, “We hold that the Jamā’ah is the truth and correct; and division is deviation and punishment.” Ibn Taymiyyah (Allah’s mercy be upon him) said, “Indeed the Jamā’ah is a mercy, and splitting is a punishment.” 

As for the [so-called] hadīth which states, “Differing in my ummah is a mercy”, then As-Subki stated, “This hadīth is not known to the scholars of hadīth, and I myself have not found for it a chain of narration that is authentic, nor weak nor fabricated.” Ibn Hazm (Allah’s mercy be upon him) said, “This hadeeth is false and a lie, made up by the people of sin.” 

[Ibn Mas’ūd: (Abu Dāwūd, no. 1920, and it is sahīh). At-Tahāwi: (Ash-Sharh At-Tahāwiyyah, 2/775). Ibn Taymiyyah: (Majmū’ Al-Fatāwā, 3/421). As-Subki (Faydul-Qadīr, 1/212). Ibn Hazm: (Al-Ahkām fī Usūlil-Ahkām, 5/61)]

Therefore, we can see here that differing is impermissible in the fundamentals of the religion, yet these innovators who follow their desires continue to split the ummah by inventing new principles into the religion which did not exist before. Our question to these innovators is 'Did the Prophet leave anything off from the religion?'. If they say yes, then this is a statement of kufr and if they say no, then they have established a proof against themself. They then claim to do it out of love for the Prophet. Do you love the Prophet more than the Sahabah did? May Allah guide them and guide us all.