Iblees was -ALWAYS- from the disbelievers

وَإِذْ قُلْنَا لِلْمَلَائِكَةِ اسْجُدُوا لِآدَمَ فَسَجَدُوا إِلَّا إِبْلِيسَ أَبَىٰ وَاسْتَكْبَرَ وَكَانَ مِنَ الْكَافِرِينَ

And [mention] when 1- We said to the angels, “Prostrate before Adam”;  so they prostrated, except for Iblees. 2- He refused 3- and was arrogant 4- and he was of the disbelievers. [2:34]

  1. Allah honored the human being by commanding the angels to prostrate before Adam. Shyatan envied Adam for this.
  2. Means to refuse the command of someone in authority over you. The refusal shows a lack of regard for the authority. Why did he refuse?
  3. He was arrogant and was seeking greatness –  his worship was not sincerely for Allah.
  4. The past tense is used instead of the present. The former implies he was and the later means he became. Shaytan was always a disbeliever and only Allah knew it.

Prophet SAW said, “No person who has the weight of a mustard seed of arrogance in his heart shall enter Paradise.”

 

[Source: BIRKAH]