Salam
Iblis tricks himself that the God of Mohammad (s) to be an evil Jinn and Iblis is the first Atheist in the sense does not believe in the Ultimate God and Creator.
Iblis recognizes that God of Mohammad (s) is right now the Lord of the worlds, but has hopes and dreams.
He believes similar to what Pharaoh says, when Pharaoh said that he would ascend and look towards the God of Musa (a), meaning reach such power of Musa (a) and defeat the genie giving him power.
Firon is in fact, one who the Qareen is Iblis himself.
So he believed Iblis and him and Shayateen would eventually get the power to reach the power of Musa (a).
Did he truly believe this? No. He lied to himself about it, but only woke up from his deception with the water was about to take him.
Iblis fears God but does not believe that the Lord of the Worlds is the Creator.
He believe the world is controlled by an evil Genie which he refers to "Allah Lord of the worlds", sarcastically, and that all miracles are power from the evil Genies.
He of course fears the power of Angels and Prophets, so when a person disbelieve in the unseen power of Prophets, he thinks they are fools and is free from them.
Also, Iblis mechanism is not through means in which a person tells himself, "I'm going to be evil for evil sake".
He deceived himself that he is all about self-love and love as opposed to judgment.
He sees judgment and hell as evil, and believes he needs to stop "Allah" lord of the worlds, from putting people in hell.
When subdued on day of judgment, he will realize "I cannot save you nor can you save me", but till that day, he has deceived himself about this.
He tells himself he can save humanity from fire and tells himself he is good and he is the hero for both Jinn and mankind.
The Jinn under him have never saved disbelievers from disasters that came to the cities, but Iblis, would say this is tactical, when he and his forces are ready, they will fight to free all humans and Jinn.
He will overthrow the order of the universe he tells himself.
His TRUE INTENTION BUT DEEP INSIDE WHERE LAYERS OF DECEPTIONS TO HIMSELF KEEP HIM FROM REALIZING, is that HE WANTS PEOPLE TO GO TO HELL WITH HIM AND HAS NO OTHER INTENTION THEN THIS.
But many Jinn and humans believe in him especially people in the occult and magic type spirituality.