الَّذِينَ كَفَرُوا وَصَدُّوا عَن سَبِيلِ اللَّهِ أَضَلَّ أَعْمَالَهُمْ

Critical Reasoning:

Warning / Threat / Consequence of an Action.

What’s the subject here?

Warning for preventing from Allah’s Path

What does a warning mean to you?

That I have to be alert, remain careful of something of which is warned about and stay out of it.

Warning to whom?

1. Kuffar
2. Whoever PREVENT from the Way of Allah

What is the Ways of Allah?

Anything and everything that Allah Subhanahu and Rasulullah Salla Allahu Alaihi Wa Sallam commanded us to do and prohibit us not to do.

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.

And already the deeds of the Kuffar has no value due to their Kufr in the Quran and in the last Messenger Muhammad, Sallallahu A’laihi Wa Sallam, in the first place (Sura Kahf 18: 103-104). Therefore, it’s not specifically applies only to them.

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! If a discouragement eventually results someone leaving/ derail from the path of Allah.

Then do you realize the impact of it?

Please explain.

Example:

Did you see people discouraging Muslims from reading the Quran with understanding and contemplating on their own by saying, “You cannot understand Quran on your own, you have to sit with an A’leem”

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.

Surely, it is better. But can all Muslims able to do it?

I don’t think so. In today’s busy time people cannot make time to do any special course to learn the Quran while most of these people regularly read the Quran without understanding.

But they have to study, understand and contemplate on the Quran, isn’t it?
Yes!

Therefore, aren’t these statements (the placing of this condition for learning the Quran under an Ulema which neither Allah Tabaraka Wa Ta’ala nor Rasulullah Salla Allahu Alaihi Wa Sallam has placed upon) discourage them to attempt understanding the Quran?

Definitely, that’s what happening with the Ummah. Only a small group learn the Quran by going to Madrasa and the rest do not make any attempt to learn the Quran.

Then, doesn’t failing to understand the Quran affect their lives and their journey to Jannah?

Sadly, the result is manifested in the condition of the Ummah.

And do not these statements then eventually prevents/ derail them from the path of Allah?

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