Since God created everything, Gen 1:1, Jn 1:3, Col 1:16, and everything he created was good, Gen 1:3, Ps 104:24, 1 Tim 4:4, it is reasonable to suppose that Satan was originally part of that good creation, but ‘fell’ into evil.
Scripture says little about this.
Isa 14:12-16 – Many interpreters have seen in this prophecy a cosmic dimension, although its primary reference is clearly to a man – the king of Babylon, Isa 14:4. The prophecy does draw on pagan mythology to depict the king’s fall from power: in one Canaanite myth a god named Athtar (meaning ‘son of Dawn’ or ‘morning star’) aspired to rule on Baal’s throne. Most modern scholars think either that the prophecy refers only to the human king, using imagery typical for that time to describe his humiliation, or that the imagery implies that the human king’s fall from power is an earthly picture of a spiritual event. In the latter case, however, it does not follow that Isaiah is referring to Satan’s original fall from innocence, but more probably to his future, final defeat.
Eze 28 – contains two prophecies against ‘the king of Tyre’. In the first, Eze 28:2-9 the king is clearly identified as a ‘man’, although he arrogantly thinks of himself as a god. But the second, Eze 28:12-18, seems from the very beginning to point beyond the human king. But the description of the king as originally wise, beautiful and blameless – and living in Eden – suggests a comparison with the fall of Adam, not of Satan. The latter interpretation is prompted by the reference in the Heb text to ‘cherub’ in Eze 28:14,16, but most scholars think that the text originally referred to the king as being ‘with’ the cherub rather than actually being a cherub.
So we know very little for sure about how Satan became a cosmic rebel. Equally, we don’t know when this took place. From Gen 3:1-7 it is reasonable to assume that this took place before the fall of humans. It is possible to speculate that Satan became jealous when God created humans in his own image and ordained them to have dominion on his behalf, but there is no way of proving it.
‘Gap theory’ suggests that Gen 1:3-31 describes a re-ordering of creation after Satan’s rebellion ruined the original creation. But the account itself does not mention anything of the sort. Isa 45:18, as gap theorists pont out, denies that God created the world ‘formless and void’ and so argue that the creation must have become so. But the passage may simply mean that God did not stop at the early stage of creation and leave it unformed and empty, but continued his creative work until it was ‘very good’. Thus, Gen 1:2 is merely describing ‘work in progress’.
See Boa & Bowman, Sense and Nonsense about Angels and Demons, 115-121.