He is a Creature
The Scripture teaches us that there is only one eternal and self-existent God who is the Creator of all things.
If Satan were not a created being then he must be eternal or self-existent, a dualism which is incompatible with what the Bible teaches about God and the world in which we live.
The Bible emphatically declares all things were created by God through Christ, and there is nothing that was not made by him – John 1:3; Col. 1:16-17 with Psa. 148:1-5; Col. 1:16; Ezek. 28:13.
He is a Spirit Being
Hebrews 1:14 describes angels as spirits and demons are called unclean spirits – Matt. 8:16; 12:45; Luke 7:21; 8:2; 11:26; Acts 19:12; Rev. 16:14.
Further, the fact we are told that “we do not wrestle with flesh and blood, but against the principalities, against the powers, against the world rulers of this present darkness, against the spiritual hosts of wickedness in heavenly places” –
Eph. 6:12 also suggest that Satan and his demons are spirit beings. Finally, the fact that Paul describes them as invisible also shows they are spirit beings – Col. 1:16.
Here it is clearly taught that the Devil and the other demons are spiritual or angelic creatures created by God in a state of innocence, and that they became evil by their own act.
It is added that man sinned by the suggestion of the Devil, and that in the next world the wicked shall suffer perpetual punishment with the Devil.
He is a Former Angel
Mention is made of the Devil in many passages of the Old and New Testaments, but The authoritative teaching of the Church on this topic is set forth in the decrees of the Fourth Lateran Council (cap. i, “Firmiter credimus”), wherein, after saying that God in the beginning had created together two creatures, the spiritual and the corporeal, that is to say the angelic and the earthly, and lastly man, who was made of both spirit and body, the council continues:
“Diabolus enim et alii dæmones a Deo quidem naturâ creati sunt boni, sed ipsi per se facti sunt mali.” (“the Devil and the other demons were created by God good in their nature but they by themselves have made themselves evil.”)
As may be gathered from the language of the Lateran definition, the Devil and the other demons are but a part of the angelic creation and their natural powers do not differ from those of the angels who remained faithful.
Like the other angels, they are pure spiritual beings without any body, and in their original state they are endowed with grace and placed in a condition of probation. It was only by their fall that they became devils.