Scripture doesn’t tell us much about Satan’s origins. He shows up abruptly in Genesis 3 as a serpent that tempts Adam and Eve into their downfall, and his presence is then assumed throughout the Bible.
Piecing together the evidence, it seems that Satan was created as an angel. Paul says that he “disguises himself as an angel of light” (2 Corinthians 11:14), and the element of disguise is located not in the fact that he appears to be an angel, but that he appears to be an angel “of light”. Even though the Bible does not answer every question that we might have about Satan’s origins, we can say a few things with certainty.
So Satan is the leader of the group of angels who rebelled against God, and he was cast out of God’s presence.
There is quite a bit of mystery here that we simply cannot know or understand, but it seems wise for us to be careful about speculating beyond those basic facts. God doesn’t tell us why he allowed the devil to rebel against him. He doesn’t tell us why he didn’t destroy Satan a long time ago.
But we must remember that, at the most basic level, he is merely a twisted part of God’s creation. He is not a competing god. Satan is still “our ancient foe”, but he is not worthy of our fear or anxiety. Jesus has dealt him a death blow (Hebrews 2:14); it’s now just a matter of waiting for Satan’s end to come.