الَّذِينَ كَفَرُوا وَصَدُّوا عَن سَبِيلِ اللَّهِ أَضَلَّ أَعْمَالَهُمْ

 Those who disbelieve and avert [people] from the way of Allah – He will waste their deeds.যারা কুফরী করে এবং আল্লাহর পথে বাধা সৃষ্টি করে, আল্লাহ তাদের সকল কর্ম ব্যর্থ করে দেন।

Critical Reasoning:

What type of sentence is this?

Warning / Threat / Consequence of an Action.

What’s the subject here?

Warning for preventing from Allah’s Path

Warning to whom?

1. Kuffar
2. Whoever PREVENT people (a Muslim) from the Way of Allah

Does the PREVENTER here indicate only/limited to the Kuffar?

It appears to be indicated only to those (who prevents) among the Kuffar.  Therefore, generally, you can apply it only to the Kuffar.
But at the same time, it could apply to anybody who prevents people from Allah’s path. Because the main issue here is the preventing from Allah’s path.
Logically, it is only the Kuffar that expected to do such things while it is unexpected that a Muslim can prevent another Muslim from Allah’s path.

What’s the Warning?

Allah will make his DEED to be lost.

What’s in it for you?

Never be in a position to prevent others from worshiping Allah Subhanahu.
Never be in a position which prevents others from working in the path of Allah

SubhanAllah!
Do you see a huge implication of this Ayah if you just stress it a little? 

How’s that?

Is discouraging someone also could be treated as preventing?

I think so!

Then, is criticizing someone’s Ebadah or work for Allah’s sake could also be treated as discouragement?

I think so!

Then do you realize the impact of it?

Please explain.

Example 1:
Does it happen very often that you make a comment or criticize someone’s work only because he is not doing the ‘best deed’, only because he is leaving the ‘best’ deed?

In Lillahi Wa Inna Ilaihi Rajiun.
Yes, that’s true! Sometimes it happens that I feel pity for a brother when I find he is busy doing something while leaving things of great value for the Ummah that he can do. But I do it only because I expect better for him and better for the Ummah.

There’s nothing wrong being expecting better deed from your brother.
But the lesson or warning from this Ayah is:

Do not let a word comes out from your mouth that might discourage him from doing what he is doing.

Astagfirullah. Yeah Allah, forgive me for my mistakes.
Aujubillah. Yeah Rabb, save me from doing this anymore.

Example 2:
Did you see people discouraging Muslims from reading the Quran with understanding?

La Hawla Wa La Quwata Illa Billah!
This has been what most of the Ulema of the Ummah have been doing.
But they have reasons. They do it to prevent people from misguidance.

But is it not discouraging people to ponder on the Quran by themselves?

They want people to learn the Quran under an Ulema

Can all Muslims do it?
When they can’t, aren’t these statements (the placing of this condition for learning the Quran under an Ulema) discourage them to attempt understanding the Quran?
And doesn’t not understanding the Quran impact their lives and their path towards Jannah?
And do not these statements then eventually prevents them to be on the path of Allah?

Inna Lillahi Wa Inna Ilaihi Rajiun.
May Allah guide these Ulema. May Allah guide the Muslims.
And Allah knows best. May Allah give us a proper understanding of our Deen. Ameen.

Critical Reasoning:

What type of sentence is this?

Warning / Threat / Consequence of an Action.

What’s the subject here?

Warning for preventing from Allah’s Path

Warning to whom?

1. Kuffar
2. Whoever PREVENT people (a Muslim) from the Way of Allah

Does the PREVENTER here indicate only/limited to the Kuffar?

Seems so. Generally, it is applicable to the Kuffar.
But at the same time, it could apply to anybody who prevents people from Allah’s path. Because the main issue here is the preventing from Allah’s path.
Logically, it is only the Kuffar that expected to do such things while it is unexpected that a Muslim can prevent another Muslim from Allah’s path.

What’s the Warning?

Allah will make his DEED to be lost.

What’s in it for you?

Never be in a position to prevent others from worshiping Allah Subhanahu.
Never be in a position which prevents others from working in the path of Allah

SubhanAllah!
Do you see a huge implication of this Ayah if you just stress it a little? 

How’s that?

Is discouraging someone also could be treated as preventing?

I think so!

Then, is criticizing someone’s Ebadah or work for Allah’s sake could also be treated as discouragement?

I think so!

Then do you realize the impact of it?

Please explain.

Example 1:
Does it happen very often that you make a comment or criticize someone’s work only because he is not doing the ‘best deed’, only because he is leaving the ‘best’ deed?

In Lillahi Wa Inna Ilaihi Rajiun.
Yes, that’s true! Sometimes it happens that I feel pity for a brother when I find he is busy doing something while leaving things of great value for the Ummah that he can do. But I do it only because I expect better for him and better for the Ummah.

There’s nothing wrong being expecting better deed from your brother.
But the lesson or warning from this Ayah is:

Do not let a word comes out from your mouth that might discourage him from doing what he is doing.

Astagfirullah. Yeah Allah, forgive me for my mistakes.
Aujubillah. Yeah Rabb, save me from doing this anymore.

Example 2:
Did you see people discouraging Muslims from reading the Quran with understanding?

La Hawla Wa La Quwata Illa Billah!
This has been what most of the Ulema of the Ummah have been doing.
But they have reasons. They do it to prevent people from misguidance.

But is it not discouraging people to ponder on the Quran by themselves?

They want people to learn the Quran under an Ulema

Can all Muslims do it?
When they can’t, aren’t these statements (the placing of this condition for learning the Quran under an Ulema) discourage them to attempt understanding the Quran?
And doesn’t not understanding the Quran impact their lives and their path towards Jannah?
And do not these statements then eventually prevents them to be on the path of Allah?

Inna Lillahi Wa Inna Ilaihi Rajiun.
May Allah guide these Ulema. May Allah guide the Muslims. 
And Allah knows best. May Allah give us a proper understanding of our Deen. Ameen.