Disbelievers of Makkah vs. Madina:
There are two groups of disbelievers that are very critical to distinguish – one in Makkah and the other in Madina. The disbelievers of Makkah, Quraysh, disbelieved so badly that they were willing to kill Prophet Muhammad (sallallahu alaihi wasallam). The other group in Madina were from the Jewish community that understood the Quran, internalized it, and yet became the worst of the enemies and were collaborating with the enemies of Islam in Makkah.

The disbelievers of Makkah were not exposed to revelation before. So, when they rejected Islam, they did it out of ignorance and arrogance. But,
- they were the first recipients of the Quran
- they received majority (two-thirds) of the Quran,
- Prophet Muhammad (sallallahu alaihi wasallam) spent most of his life with them even before becoming a Messenger.
So, these people had the best opportunity to accept the religion of Allah (subhanahu wa ta’ala) than any other nation on the face of this earth and yet they were willing to kill the Messenger of Allah (sallallahu alaihi wasallam).
On the other hand, the disbelievers of Madina had a background in revelation. But,
- they already hate Prophet Muhammad (sallallahu alaihi wasallam) because he was not from their race.
- they didn’t even spare their own books in the past. They did kufr of the akhirah and of their own Prophets in their own books. They put so much effort into erasing the concept of akhirah from their books and Quran was just hitting them back with it by talking about akhirah over and over.
Hence, the disbelievers of Madina demonstrated disbelief not just in front of Prophet Muhammad (sallallahu alaihi wasallam), but they have been demonstrating disbelief from before.
So, this ayat applies well to both groups of people because it didn’t even matter had Prophet Muhammad (sallallahu alaihi wasallam) warned them or not, they were not going to believe.
The Gravity of Being Called a Kafir in the Quran:
When we read “Allazeena kafaroo” in the Quran, it is context based and most of the time it is referring to very, very bad people. It is not referring to just any non-Muslim. It refers to the worst of them, the ones who have been most stubborn, who don’t want to budge from their position. It is not a light term in the Quran. When Allah (subhanahu wa ta’ala) calls someone kafir, it is one of the worst words used for someone.

Allah (subhanahu wa ta’ala) used the phrase “had you warned them or not”. Prophet Muhammad (sallallahu alaihi wasallam) wouldn’t just warn. He would teach, preach, give good news etc. too. But the word used here is “’a-anzar tahum“, which refers to “warning”. Warning is done for punishments, for consequences. The ultimate consequences for the believers are in the akhirah. Hence, the language is perfectly structured referring to those who have denied the akhirah.
Meaning
No doubt the certain group of people that have disbelieved, their eventual consequence (or outcome) will be the same, had you, Prophet Muhammad (sallallahu alaihi wasallam), warned them, or had you not warned them. They do not believe and they will not believe.