The Hadith: (Full references and Arabic text is below)

“Never a Prophet had been sent before me by Allah towards his nation who had not among his people (his) disciples and companions who followed his ways and obeyed his command.
Then there came after them their successors who said whatever they did not practise and practised whatever they were not commanded to do.
He who strove (JIHAD) against them with his hand was a believer:
he who strove against them with his tongue was a believer,
and he who strove against them with his heart was a believer
and beyond that there is no faith even to the extent of a mustard seed”.

My Critical Reasoning:
Part 1: The PRACTICE of Deen has changed long back:

“Never a Prophet had been sent before me by Allah towards his nation who had not among his people (his) disciples and companions who followed his ways and obeyed his command.

Then there came after them their successors who said whatever they did not practice, and practiced whatever they were not commanded to do.”

The sunnah of Allah Subhanahu is always same! The sunnah of believers was always same (this Hadith).
So, what is the Sunnah of the believers that Rasulullah sallillahu alaihi wa sallam warned here?
That after the followers (sahaba), there will be successors,
What are the characteristics mentioned of the believers (the successors)?
That they will say what they will not practice,
That they will practice what was not practised before (Bida’a)
Now, was the people before a different species than the people who followed Rasulullah?
They are the same species.
What does that suggest? How long it took for the followers of Eisa alaihi wa sallam after him to change what Eisa alaihi wa sallam taught?
Not even 100 years.
Therefore, as per sunnah of the believers what is supposed to happen? How come you think the practice of Islam has remained the same as it was at the time of Rasulullah sallillahu alahi wa sallam or at the time of Sahaba? Rather the Hadith of Hudaifah (his question) also provide an added evidence to that, change of people and change of those people who will call from the gate of Jahannam.

I don’t mean here that the books have been changed. I mean the understanding of the books and practice of people must have changed.

Look at the Salat. I never thought that Salat remains the same after reading the books of Salat. Evidence? Rasulullah sallillahu alaihi wa sallam used to pause after every ayah and when He recited an Ayah about Jahannam, He paused and asked refuge with Allah from it and when He recited an Ayah about Jannah, He paused and asked Allah to grant him Jannah. But In my life, I didn’t see any imam around the world doing it.

And then the change of Allah’s system, slavery is made Haram what Allah and Rasul made Halal and Sahaba practised it. But today’s Ulema mentions it with an apologetic notion. That’s how much Muslims minds were shaped by the system of kuffar’s, by the philosophy and ways of life of kuffar’s.

Who made it, how did that happen?
Ulema that preaching voting and “no Khilafah is needed”
Ulema that made Riba Halal by the name of ‘Islamic’ which finally has a link with Riba.
The conclusion: The PRACTICE of deen has CHANGED! May Allah guide Muslims! Ameen.

AND ALLAH KNOWS BEST

Part 2:

Point 1: The practice of Deen of Allah has changed
So, we know from this Hadith that The DEEN of Allah must have changed by the successors of His companions.
You might not see the change in your own eyes because the change is not rapid, it took generations to change to occur thus it doesn’t appear in one’s lifetime to experience the change himself but this Hadith says that the change will occur and brought in by the followers of the sahaba, therefore, it must have happened long before, no doubt about that, if you believe this hadith is a truth.

Point 2: Correcting Kuffar’s Mindset: “Hate the Criminal not the crime”
The believers have to do Jihad against those who do it (the change of deen). And the lowest act of Jihad is disliking, hatred in the HEART against THEM (not against IT) (We Muslim hate the criminal and the crime both, NOT “Hate the Crime, not the Criminal”, which is a Kuffar’s philosophy)

Point 3: It’s a simple math! Correcting Kuffar’s Mindset: “You can’t Judge” 
Does this Hadith give you a tool to understand the Eeman of a Muslim?
Yes, it does.
How can you understand their level of faith?

When you see someone in authority who prevent or doesn’t allow occurring a change of deen (not limited to Bida’a as usually we end up only limiting CHANGE OF DEEN to Bida’a in rituals, in fact this change of deen happened in every aspect of deen which is not only turned to Bida’a but to SHIRK and KUFR), would you not understand his level of Eeman?
Then someone who voices against it, talks about it, write about it, discuss it, take attempts in whatever way is possible, you can understand his level of Eeman, don’t you?
But what about someone’s heart? How would you measure what’s in someone’s heart?
Well, people reflect their heart in their words of mouth, in their facial expressions, through their glances, through their body languages. Don’t you understand the eye of a person who hates you by the look he gives to you? Don’t you understand the facial expression of your parent/teacher/supervisor/boss when he is not happy with you?

Point 4: Enjoining Good and Forbidding Evil:
See that’s how it is so important in Islam to enjoin good and forbid evil, knowing the falsehood and hating it hard! It is simply standing for the TRUTH and HATEING the falsehood. This is one of the basic Mindset of being a Muslim. You have to have strong adherence to the RIGHT and bitter HATRED for the wrong.

Point 5: No Knowledge of DeeN means NO FAITH
To understand the CHANGE of something, is it logical that someone MUST KNOW the CORRECT thing in the first place?
Yes, I think so. That seems obvious!
Therefore, to understand the CHANGE of DEEN a believer MUST KNOW the deen first, right?
Absolutely, no doubt about that.
But did we not miss something here?
Not only knowing the deen but have to know the CORRECT deen!
What is the CONSEQUENCES for not being able to do the LOWEST JIHAD (which is by Jihad by heart)
“He has NO FAITH to the extent of a mustard seed.”
Then let’s do the simple math:
No knowledge of correct deen = Not able to FORBID evil = NO FAITH (not even to the extent of a mustard seed)
What does that suggest?
If you do NOT STUDY deen, you have NO FAITH.
It’s a simple math.

Point 6: Rasulullah might not able to pull you out from Jahannam
What a warning for those who remain bystander while watching a change of deen.
Do you know the consequence of it?
In case he ends up in Jahannam, he won’t be pulled out from Jahannam with those when Allah will command Rasulullah Salla Allahu alaihi wa Sallam to pull out those from Jahannam who has ‘mustard-seed’ of faith in their heart! (Hadith*) because he does not have the “mustard seed of faith” in him unless he doesn’t enjoin good and forbid evil.
O Allah, save Muslims to remain a bystander to change of deen. Ya Rabb, give me knowledge, wisdom and strength to stand by the truth and fight against the falsehood. Ameen.

The detail of the Hadith:
It is narrated on the authority ‘Abdullah b. Mas’ud that the Messenger of Allah (may peace and blessings be upon him) observed:
Never a Prophet had been sent before me by Allah towards his nation who had not among his people (his) disciples and companions who followed his ways and obeyed his command.
Then there came after them their successors who said whatever they did not practise and practised whatever they were not commanded to do.
He who strove (JIHAD) against them with his hand was a believer:
he who strove against them with his tongue was a believer,
and he who strove against them with his heart was a believer
and beyond that, there is no faith even to the extent of a mustard seed.
Abu Rafi’ said: I narrated this hadith to ‘Abdullah b. ‘Umar; he contradicted me. There happened to come ‘Abdullah b. Mas’ud who stayed at Qanat, and ‘Abdullah b ‘Umar wanted me to accompany him for visiting him (as ‘Abdullah b. Mas’ud was ailing), so I went along with him and as we sat (before him) I asked Ibn Mas’ud about this hadith. He narrated it in the same way as I narrated it to Ibn ‘Umar.

حَدَّثَنِي عَمْرٌو النَّاقِدُ، وَأَبُو بَكْرِ بْنُ النَّضْرِ وَعَبْدُ بْنُ حُمَيْدٍ – وَاللَّفْظُ لِعَبْدٍ – قَالُوا حَدَّثَنَا يَعْقُوبُ بْنُ إِبْرَاهِيمَ بْنِ سَعْدٍ، قَالَ حَدَّثَنِي أَبِي، عَنْ صَالِحِ بْنِ كَيْسَانَ، عَنِ الْحَارِثِ، عَنْ جَعْفَرِ بْنِ عَبْدِ اللَّهِ بْنِ الْحَكَمِ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ الْمِسْوَرِ، عَنْ أَبِي رَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ مَسْعُودٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ

‏ “‏ مَا مِنْ نَبِيٍّ بَعَثَهُ اللَّهُ فِي أُمَّةٍ قَبْلِي إِلاَّ كَانَ لَهُ مِنْ أُمَّتِهِ حَوَارِيُّونَ وَأَصْحَابٌ يَأْخُذُونَ بِسُنَّتِهِ وَيَقْتَدُونَ بِأَمْرِهِ ثُمَّ إِنَّهَا تَخْلُفُ مِنْ بَعْدِهِمْ خُلُوفٌ يَقُولُونَ مَا لاَ يَفْعَلُونَ وَيَفْعَلُونَ مَا لاَ يُؤْمَرُونَ

فَمَنْ جَاهَدَهُمْ بِيَدِهِ فَهُوَ مُؤْمِنٌ

وَمَنْ جَاهَدَهُمْ بِلِسَانِهِ فَهُوَ مُؤْمِنٌ

وَمَنْ جَاهَدَهُمْ بِقَلْبِهِ فَهُوَ مُؤْمِنٌ

وَلَيْسَ وَرَاءَ ذَلِكَ مِنَ الإِيمَانِ حَبَّةُ خَرْدَلٍ ‏”

‏ ‏.‏ قَالَ أَبُو رَافِعٍ فَحَدَّثْتُهُ عَبْدَ اللَّهِ بْنَ عُمَرَ فَأَنْكَرَهُ عَلَىَّ فَقَدِمَ ابْنُ مَسْعُودٍ فَنَزَلَ بِقَنَاةَ فَاسْتَتْبَعَنِي إِلَيْهِ عَبْدُ اللَّهِ بْنُ عُمَرَ يَعُودُهُ فَانْطَلَقْتُ مَعَهُ فَلَمَّا جَلَسْنَا سَأَلْتُ ابْنَ مَسْعُودٍ عَنْ هَذَا الْحَدِيثِ فَحَدَّثَنِيهِ كَمَا حَدَّثْتُهُ ابْنَ عُمَرَ ‏.‏ قَالَ صَالِحٌ وَقَدْ تُحُدِّثَ بِنَحْوِ ذَلِكَ عَنْ أَبِي رَافِعٍ ‏.‏

Reference

Sahih Muslim 50 a
In-book reference : Book 1, Hadith 86
USC-MSA web (English) reference : Book 1, Hadith 81
(deprecated numbering scheme)