Satan (which means "adversary" or "accuser") was originally created as a holy angel known as Lucifer (which is the latin name for "Morning Star" and "light bearer"), who belonged to the Cherubim choir of Angels.

In his arrogance, satan decided to no longer serve God, but to take God's place as the following verses state. "How you have fallen from heaven, O star of the morning, son of the dawn! You have been cut down to the earth, You who have weakened the nations! [13] "But you said in your heart, 'I will ascend to heaven; I will raise my throne above the stars of God, And I will sit on the mount of assembly In the recesses of the north. [14] 'I will ascend above the heights of the  clouds; I will make myself like the Most High.'" (Isaiah 14:12-14).

[7] And there was war in heaven. Michael and his angels fought against the dragon, and the dragon and his angels fought back. [8] But he was not strong enough, and they lost their place in heaven. [9] The great dragon was hurled down — that ancient serpent called the devil, or Satan, who leads the whole world astray. He was hurled to the earth, and his angels with him. (Revelation 12:7-9)

Our Lord confirmed this when He said, "I saw Satan fall like lightning from heaven" (Luke 10:18).

satan is not strong enough to attack God directly. God is far too powerful. It would be like an ant trying to destroy the sun. Even that is a very poor comparison. So, satan directs his attacks against mankind (which is created in God's image), and against the rest of God's creation.

"Woe to the inhabitants of the earth and the sea! For the devil has come down to you, having great wrath, because he knows that he has a short time" (Revelation 12:12).

“Be sober, be vigilant; because your adversary the devil walks about like a roaring lion, seeking whom he may devour.” (1 Peter 5:8)

satan's primary mission is to take as many souls to hell with him as he can. He hates God. He hates mankind. He hates the fact that Jesus Christ, the Son of God Himself came to Earth to suffer and die for us. He hates the fact that Jesus defeated him and paid the price of our sins with His Sufferings and death on the Cross.

Christians don't need to be afraid of satan, because Our Lord said; “Behold, I give you the authority to trample on serpents and scorpions, and over all the power of the enemy, and nothing shall by any means hurt you” (Luke 10:19). Our Lord gave his followers the authority to use His name to repel and drive out satan and his cohorts. "[17] And these signs will accompany those who believe: In my name they will drive out demons; they will speak in new tongues;" (Mark 16:17)

[11] Put on the full armor of God so that you can take your stand against the devil's schemes. [12] For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms. [13] Therefore put on the full armor of God, so that when the day of evil comes, you may be able to stand your ground, and after you have done everything, to stand. [14] Stand firm then, with the belt of truth buckled around your waist, with the breastplate of righteousness in place, 15] and with your feet fitted with the readiness that comes from the gospel of peace. [16] In addition to all this, take up the shield of faith, with which you can extinguish all the flaming arrows of the evil one. [17] Take the helmet of salvation and the sword of the Spirit, which is the word of God. 18And pray in the Spirit on all occasions with all kinds of prayers and requests. With this in mind, be alert and always keep on praying for all the saints. (Ephesians 6:11-17)