Allah said, "those who were given the Scriptures did not differ except out of mutual jealousy and hatred (bagyan) after knowledge had come to them." Informing that their differing arose after the coming of knowledge that made clear to them as to what they should steer clear of - for indeed Allah does not let a people become misguided after He guided them except after telling them what they should steer clear of. And informing that they differed only due to the reason of mutual jealousy and hatred. Al-Bagy means to exceed the proper bounds as was indicated by ibn Umar (in his commentary to the verse): 'arrogance and jealousy' - this arising due to differing ijtihaads that were not based upon knowledge. What is not meant by al-Bagy is the permissible difference that arises between the scholars for al-Bagy is either the neglect or omission of the truth, or exceeding the proper bounds. Exceeding the proper bounds either by leaving an obligatory action or by performing a forbidden action and it is known that splitting and disunity is the outcome of this.
An example of this lies in what He said about the People of the Book,
"and from those who call themselves Christians, We took their covenant but they abandoned (or forgot) a portion of the Message that was sent to them. So we planted enmity and hatred till the Day of Resurrection." (5:14)
So He informed us that in their forgetting/abandoning a portion of the Message - meaning leaving a portion of that which they were commanded with - lay the reason for their enmity and hatred.
This is what is occurring amongst the adherents of our religion and it can be found amongst the various parties that differ with regards to their principles and many subsidiary matters - found amongst the scholars and the worshippers - to the point that one finds a resemblance to the two nations about whom one says about the other: 'They are upon nothing.'
This is like what we find amongst the legal jurists who stick to the outward actions of the religion, and those that take to the path of tasawwuf who stick to the inward actions of the religion for each one of these negates the path of the other and claims that they are not from the People of the Religion, or they oppose the other in a way of opposing of those who do not consider the other to be part of the religion. There occurs between them enmity and hatred.
This is because Allah has ordered that the heart be purified just as He has ordered that the body be purified. Both these aspects of purification are from the religion that Allah has commanded and made obligatory. Allah said,
"Allah does not want to place you in difficulty, but He wants to purify you and to complete His Favour on you that you may be thankful." (5:6 - the verse pertaining to wudu)
"In it (the mosque) are men who love to purify themselves and Allah loves those who purify themselves." (9:108)
"Indeed Allah Loves those who turn to Him in repentance and He loves those who purify themselves." (2:222)
"Take charity from their wealth in order to purify and sanctify them." (9:103)
"They are the ones whose hearts Allah does not wish to purify." (5:41)
"Indeed the polytheists are impure and filthy" (9:28)
So we find that the concern of many of the legal jurists and the worshipper is only the purification of the body, and he goes beyond the requirement of the sharee'ah with respect to his concern and action. And he leaves what he has been commanded and recommended with respect to the purification of the heart, and he does not understand purification except in this bodily aspect. And we find that the concern of many of the sufis is the purification of the heart only, to the extent that he will go beyond the requirement of the Sharee'ah with respect to his concern and action. And he leaves what he has been commanded and recommended with respect to the purification of the body.
So the first group go to the extreme of wasting water and considering to be impure that which is not impure, and staying away from that which has not been legislated to stay away from, this despite their hearts containing different shades of jealousy, arrogance and hatred towards their brothers. In this there is a clear resemblance to the Jews.
The other group goes to the extreme of negligence, they go to great extents to safeguard the heart to the point that they consider ignorance of that which is obligatory to know - such as the evil from which it is obligatory to steer clear of - from those things that safeguard the heart (from actually desiring that evil). In this they do not differentiate between safeguarding the heart from desiring evil, and safeguarding the heart through its knowing the evil and its knowing that with which it has been commanded. Then alongside this negligence and ignorance they do not preserve themselves from the impure things and therefore establish the obligatory purification in the way of the Christians.
So enmity has occurred between these two groups due to their leaving a portion of what they have been commanded with, and due to their bagy - which is to exceed the correct bounds, either by ignoring or omitting the truth, or by enmity and oppression. Bagy sometimes occurs amongst some people, or sometimes with regards to the rights of Allah - both of these are implied. This is why He said, "out of mutual jealousy and hatred," for each group was displaying bagy to the other by not acknowledging the others right and not refraining from showing enmity to the other. Allah said,
"Nor were the People of the Book divided until the clear proof came to them." (98:4)
"Mankind was one nation and Allah sent Messengers as bringers of glad-tidings and warners. And He revealed with them the Book in truth so that they may judge amongst mankind in that which they differed. And only those to whom it was given differed concerning it after clear proofs came to them through hatred, one to another." (2:213)
"Indeed those who divide their religion into sects, you have no concern in them in the least." (6:159)
"(Always) turning in repentance to Him, and be afraid an dutiful to Him: and establish the prayers and do not be of the polytheists - of those who split up their religion and became sects, each sect rejoicing in that which is with it." (30:31-32)
This is because each party of the polytheists used to worship whatsoever god their desires dictated as Allah said, "what you call the polytheists to is hard upon them."
"O Messengers! Eat of the goodly things and do righteous deeds. Indeed I am Well-Acquainted with what you do. And Verily! This, your religion, is one religion and I am your Lord, so keep your duty to Me. But they have broken their religion into sects, each group rejoicing in what it has." (23:51-52)
So it becomes clear the way of unity is by means of gathering all of the religion, and acting upon it in its entirety - that is worshipping Allah Alone without any partners as He ordered, inwardly and outwardly.
The way to disunity is by leaving a portion of that which the servant has been commanded with, and by al-bagy.
The result of unity is: the Mercy and Pleasure of Allah, His sending salaah upon the servants, and happiness in this world and in the hereafter, and white faces (on the Day of Judgement)
The result of disunity is: the Punishment of Allah, His Curse, and black faces, and the Messenger (SAW) declaring himself free of them.
This is one of the proofs that consensus is a definitive proof, for when they unite then they are all obeying Allah in this and have Mercy showered on them. And there cannot be obedience to Allah and His Mercy by way of an action that He has not commanded, in either belief, saying or action. For if there were to be a saying or action upon which they agreed that had not been commanded by Allah, then this would not be obedience to Allah and neither would it be a means of His Mercy. Abu Bakr al-Azeez depended upon this proof in the beginning of his 'at-Tanbeeh.'
Taken from 'Majmoo Fataawaa ' (1/14 -17) of Shaykh al-Islaam ibn Taymiyyah.