Not Judging by the Book of Allāh: A Cause for Calamities to Strike the Ummah
Understand that judging by what Allāh revealed in His Book is incumbent upon the Muslim, and not doing so results in many calamities befalling the Ummah.
That there would always continue to be a group from this Ummah clinging to the truth and to the pure Tawḥīd of Allāh until the day of Judgment.
Shaykh Muḥammad ibn Jamīl Zaynū
Allāh - the Most High - said:
“Say: Allāh has power to send punishment upon you from above you, or from under your feet; or to cover you in confusion with party strife, and make you taste the inter-fighting and violence of one another. See how We variously explain the Āyāt (proofs and signs), so that they may understand.” [Sūrah al-Anʿām 6:65]
Allāh’s Messenger (ṣallallāhu ʿalayhi wa-sallam) said, when this verse was revealed: “Say: Allāh has power to send punishment upon you from above you.” So he said: “I seek refuge in Your Face.” “Or from under your feet.” He said: “I seek refuge in Your Face.”
“Or to cover you in confusion with party strife, and make you taste the inter-fighting and violence of one another.” So Allāh’s Messenger (ṣallallāhu ʿalayhi wa-sallam) said: “This is lighter or easier.” 
The Prophet (ṣallallāhu ʿalayhi wa-sallam) said: “Indeed Allāh gathered up the earth for me so that I saw its eastern and western parts, and indeed the dominion of my Ummah will reach what was gathered up for me from it. And I have been granted the two treasures of gold and silver. And indeed I asked my Lord for my Ummah that it should not be totally destroyed by a prevailing famine, nor to allow an enemy to totally conquer it, except from amongst themselves. So my Lord said: O Muḥammad! When I decree a matter, then it is not reversed. I grant to you for your Ummah that it would not be totally destroyed by a prevailing famine, nor will it be totally conquered by an external enemy, even if all the notions were to rally against it. However, a group from your Ummah will destroy one another and imprison one another.” 
The Prophet (ṣallallāhu ʿalayhi wa-sallam) also said: “I asked my Lord for three things, I was granted two; but prevented from one: I asked my Lord that He should not destroy my Ummah with drought and famine, so He granted it to me. I asked my Lord that He should not destroy my Ummah with drowning, so he granted it to me. And I asked Him not to cause my Ummah to fight amongst themselves, but he refused that to me.” 
And this inter-fighting that has befallen the Muslims has been further emphasized in his (ṣallallāhu ʿalayhi wa-sallam) saying: “And when your leaders do not judge by the Book of Allāh, not seeking the good and welfare with Allāh has revealed, then Allāh would cause them to fight one another.” 
The Prophet said, in an addition to the second narration: “Indeed, what I fear for my Ummah are misguided leaders, and that when the sword is drown out against them, it will not be lifted from them until the Day of Judgement. And the Hour will not come, until sections of my Ummah will follow the mushrikīn (those who worship others along with Allāh), and until many sections of my Ummah will worship idols. And there will be in my Ummah thirty liars; all of whom will claim that he is a Prophet. But I am the last and the seal of the Prophets; there is no Prophet after me. And there will not cease to be a group from my Ummah being helped upon the truth, not being harmed by those who oppose them; until the command of Allāh comes.” 
SOME BENEFITS FROM THE ABOVE ĀYAH AND ḤADĪTH:
: It shows the great power of Allāh - the Most High – in sending punishment to the previous nations, from above them, and from beneath their feet, causing them to be annihilated and destroyed. Due to this, Allāh’s Messenger (ṣallallāhu ʿalayhi wa-sallam) sought refuge and protection in the Face of his Lord against his Ummah being punished in a similar manner.
: The first ḥadīth (narration) affirms that Allāh – the Most High – has a wajh (Face), but in a manner that befits His Greatness and Majesty, there being any tashbīh (resemblance) between Allāh and between that of His creation; as Allāh said: “There is none like Him; and He is the all-Hearing, the all-Seeing.” [Sūrah al-Shūrá 42:11]
: It shows the great power of Allāh - the Most High - in that whenever the Muslims abandon judging by the Sharīʿah (Divinely Prescribed Law) of Allāh; but rather take to manmade laws that oppose the Law of Allāh - which, with deep regret, is happening now - then Allāh causes the Ummah to split-up into sects and parties, and makes them fight one another.
: It Shows the might of Allāh in that He gathered-up the earth for His Messenger (ṣallallāhu ʿalayhi wa-sallam), so that he saw its eastern and western parts; and that the dominion of his Ummah would extend over all that which was gathered-up for him, from it.
: It shows the mercy of Allāh to the Muslim Ummah, in His not sending against them a punishment that would totally destroy the entire Ummah; but it would only cause destruction to parts of the Ummah.
: An-Nawawee (d.676H) - raḥimahullāh - said: “In this ḥadīth is a clear and apparent miracle, all of which has come to pass - so all praise be to Allāh.” 
: It also shows the mercy of Allāh to the Muslim Ummah in that He has not allowed an external enemy to overpower them, such that they have complete control over them. Rather, Allāh has caused some parts of the Ummah to fight against others; and this is far easier than the former case.
: It Shows the fear that Allāh’s Messenger (ṣallallāhu ʿalayhi wa-sallam) had for his Ummah with regards to the misguided leaders who would not take hold of the Book and the Sunnah; and how he warned against them.
: That when the sword has been drawn out against this Ummah, then it would not be lifted from it until the Day of Judgement - as is happening at this very time now.
: It proves that shirk would be present in the Ummah. And shirk is: Directing worship, or forms of worship, to other than Allāh, such as making duʿāʾʾʾ (supplication) to other that Allāh, or judging by other than what Allāh has legislated and revealed.
: We are informed about the existence of those who would claim that they are Prophets and that they are all liars in this claim; such as Mūsáylimah al-Kadhdhaab in earlier times, and in more recent times Ghulām Aḥmad Mirza, the Qaadiyaanee (whose followers are called Qaadiyaanees).
: That there would always continue to be a group from this Ummah clinging to the truth and to the pure Tawḥīd of Allāh until the day of Judgement.
Imām al-Nawawī - raḥimahullāh - said:
“As for this group, then Imām al-Bukhārī (d.256H) said: “They are the people of knowledge.” And Imām Aḥmad Ibn Ḥanbal (d.241H) said: “If they are not Ahlul-Ḥadīth (the Scholars of Ḥadīth), then I do not know who they can be.” Qaadee ʿIyāḍ said: “What Imām Aḥmad meant by that was Ahl al-Sunnah wa-al-Jamāʿah and whosoever holds the ʿaqīdah (belief) of Ahl al-Ḥadīth.” I say: It is possible that this group could be scattered amongst the various categories of Believers, from them:- the brave warriors, the Scholars of Ḥadīth, the zuhhād (those who abstain from the world in a praiseworthy manner), those that enjoin the good and forbid the evil, and other categories of people who do good.” 
Ibn al-Mubārak (d.181H) - raḥimahullāh - said: “In my view they are Aṣḥāb al-Ḥadīth (the companions of Ḥadīth).” 
So the preferred saying is that this Aided and Victorious Group are those that act in accordance with the authentic ḥadīth, not giving precedence to the saying of anyone over and above the saying of Allāh and His Messenger (ṣallallāhu ʿalayhi wa-sallam).
 Tawjīhul-Muslimīn ilal-Ṭarīqin-Naṣr wa-al-Tamkīn (p. 49-53)
 Related by al-Bukhārī (no. 4628), from Jābir (raḍī Allāhu ʿanhu)
 Related by Muslim (no. 2889), from Thawbān (raḍī Allāhu ʿanhu)
 Related by Muslim (no. 2216), from Saʿd Ibn Abī Waqqās (raḍī Allāhu ʿanhu)
 Ḥasan: Related by Ibn Majāh (no. 4019), from Ibn ʿUmar (raḍī Allāhu ʿanhu) It was authenticated by al-Albānī in al-Ṣaḥihah (no. 106).
 Ṣaḥīḥ: Related by Abū Dāwūd (no. 3952), from Thawbān (raḍī Allāhu ʿanhu) It was authenticated by al-Ḥāfiẓ Ibn Ḥajar in al-Matālibul-ʿĀliyah(4/264).
 Sharḥ Ṣaḥīḥ Muslim (1/234)
 Sharḥ Ṣaḥīḥ Muslim (13/67)
 Related by al-Khaṭīb al-Baghdādī in Sharaf Aṣḥāb al-Ḥadīth (p .26)