God has one Purpose. God wants to bring more sons to glory. God, His Word, and His Spirit in complete harmony decided things of utter importance about this purpose. The Bible is silent on when this happened, but the all-knowing Omniscient God knew beforehand a rebellion of one of His spirit-beings and his (its) attempt to foil the purpose of God by deceiving mankind to doubt and disobey God. This foreknowledge, in effect, moved God to plan His course of action for the fulfillment of His purpose even before anything happened.
God set His plan rolling. Then came into existence matter – the earth and its foundation in the heavens.

“Then the morning stars sang together, and all the sons of God shouted for joy”- Job 38:7.
Morning Stars. Sons of God. All Spirit beings. All, created by God. All, created by His Word. Even before He created anything.

Are Angels eternal? All angels were created by God as spirit beings. So, all of them had a beginning. Therefore, they are not eternal with respect to the past. But they don’t have an end – they will exist forever (like how man will also exist forever). They were created by God to do His bidding. Some of His angels are made winds, and some, flames of fire. They can accomplish mighty things by the power of God made available to them by God Himself. One such angel slew three lakh soldiers one night and another slew all first-born of Egypt overnight. They are messengers, bringing messages from God to mankind. (Anjal in Thamizh means message, letter, post, or courier). While they are called sons of God because they were created by Him, they are also meant to be ministering spirits for the children of God. They are sent to protect His people but also destroy those who rebel against God.

One such “star” wanted to break free. One of God’s creations wanted to raise above his Creator. One of the sons of God had occupied so prominent a position in the court of God that pride entered him. He thought he could subdue his Creator. Fool! The spirit-being who revolted against the authority of God became Satan, God’s adversary, from then on. He was (unfortunately!) nowhere near when God purposed and planned His course.

This star gathered a significant number of angels around him to support his intentions – one-third of the spirit beings. He deceived them and made them believe that he would succeed in this endeavor. The Bible calls this personality Satan – the enemy. He is the adversary of God. He is also called the adversary of the believing kind. Be sober, be vigilant; because your adversary the devil, as a roaring lion, walks about, seeking whom he may devour: - 1 Pet 5:8. He is a liar and a murderer from the beginning and is the father of lies. He is also called the prince of this world/age, the Devil, the evil one, the Old Serpent, the wicked one, the accuser, and the Dragon.

This Satan has sinned from the beginning – even before time began.

Is Satan eternal? No. Satan is a spirit being and all spirit beings were created by God, the Creator of everything. So, Satan is also a creation of God. He certainly had a beginning. So, He is not eternal. But he was not created as the evil one. He became the evil one sometime (?) after he was created. However, every spirit being and all that has a spirit (man too!) will remain eternal in the future – even beyond time. All of God’s people will be blessed with eternal life. All God’s adversaries will be condemned to eternal death. That includes God’s prime enemy – Satan. Some believe, a loving God would forgive His enemy. He will not.

Satan’s final destiny is sealed in His-story. And Satan knows it. And, that is why he is desperate, nervous, and terrified and is trying all that he can to foil the plan and upset the purpose of God.