Answer
Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear brother, we really appreciate the keenness you show in pursuing knowledge about Islam. And we really consider it a privilege to have you as a regular visitor to our site.
In his response to your question, we’d like to cite a fatwa issued by Sheikh M. S. Al-Munajjid, a prominent Saudi Muslim lecturer and author, in which he states,
Allah says, “He (Iblis) was one of the jinn; he disobeyed the Command of his Lord…” (Al-Kahf 18: 50)
The apparent meaning of this verse is that the reason for his disobedience of his Lord was the fact that he was one of the jinn. The Arabic conjunction ‘fa’ in the Arabic text of the verse “kana min al-jinni fa-fasaqa `an amri rabbihi” is indicative of the reason, as in the Arabic phrase “saha fa sajada” which means, “he forgot (in prayer), so he prostrated”, i.e., that was because he forgot.
Similarly, the phrase “kana min al-jinni fa-fasaqa `an amri rabbihi (he was one of the jinn [so] he disobeyed…)” means that this was because he was in essence one of the jinn, because this feature is what made him different from the angels, for they obeyed the command but he disobeyed.
Because of the apparent meaning of this Quranic verse, a number of scholars were of the opinion that Iblis was not originally one of the angels, rather he was one of the jinn, but he used to worship Allah with them, so he was called by their name because he followed them, just as an ally of a tribe may be given their name. The dispute as to whether Iblis was originally an angel whom Allah changed into a devil, or whether he was not originally an angel but was included in the word mala’ikah (angels) because he had joined them and worshipped Allah with them, is a well-known dispute among the scholars. The evidence of those who say that he was not originally one of the angels is based on two things:
1– The fact that angels are protected against committing kufr (disbelief) as was committed by Iblis, as Allah says about them, “…who disobey not, (from executing) the Commands they receive from Allah, but do that which they are commanded.” (At-Tahrim 66:6)
“They speak not until He has spoken, and they act on His Command.”(Al-Anbiya’ 21:27)
2– Allah clearly states in the aforementioned verse (Al-Kahf 18 :50) that he was one of the jinn, and the jinn are not angels.
Among those who stated that he was not originally one of the angels based on the apparent meaning of this verse was al-Hasan al-Basri, who was supported by az-Zamakhshari in his Tafsir (exegesis)
Al-Qurtubi said in his commentary on surat al-Baqarah: “The idea that he was one of the angels is the view of the majority, Ibn `Abbas, Ibn Mas`ud, Ibn Jurayj, Ibn al-Musayyib, Qatadah and others. It is the view chosen by ash-Shaykh Abu al-Hasan and regarded as more correct by at-Tabari, and it is the apparent meaning of the phrase “except Iblis” (al-Kahf 18:50)
The clearest evidence concerning this matter is the one cited by those who said that he was not an angel based on the Quranic verse that reads, “He (Iblis) was one of the jinn; he disobeyed the Command of his Lord…” (Al-Kahf 18:50)
Allah Almighty knows best.
Excerpted, with slight modifications, from: www.islamqa.info