Satan is a fallen angel. He was created as a cherub to worship and serve Almighty God (Ezekiel 28:12–14). So every general truth regarding angels is true of Satan. The holy angels obey the Lord, and Satan and his angels rebelled against Him and live in a constant state of mutiny. Angels are spirits (Hebrews 1:14). Being one of the angelic hosts, Satan is also a spirit.
Satan is called the “god of this world” and the “prince of the power of the air” (John 12:31; 2 Corinthians 4:4; Ephesians 2:2). Those titles hint at a realm beyond our understanding, a realm where all players are spirits and just as real as our material world. When Satan rebelled against God, he was judged and cast down to the earth. Many rebellious angels went with him. They are most likely what we now know as demons. As spirits, they can inhabit living creatures such as humans and animals (Mark 5:1–13). Jesus spoke often of unclean spirits and cast them out of people who were tormented by them (Mark 3:11; Matthew 12:43; Luke 9:42–43).
As spirits, Satan and his demons can work undetected around and within us. Ephesians 6:16 calls Satan’s attacks “flaming arrows” or “fiery missiles.” Even Peter, one of the Lord’s trusted disciples, was influenced by a satanic spirit when he contradicted Jesus’ plan of going to the cross (Matthew 16:22–23). Jesus rebuked him by saying, “Get behind me, Satan!” Peter was completely unaware of the influence Satan was having on his words. Peter, who knew the tricks of Satan firsthand, later wrote, “Be sober-minded; be watchful. Your adversary the devil prowls around like a roaring lion, seeking someone to devour” (1 Peter 5:8).
“Though we walk in the flesh, we are not waging war according to the flesh. For the weapons of our warfare are not of the flesh but have divine power to destroy strongholds” (2 Corinthians 10:3–4). Because Satan is spirit, we must battle him in spirit. Earthly weapons are of no use in defending our hearts and minds against such an adversary. However, God equips His children with His own armor, and we have the sword of the Spirit, which is the Word of God (Ephesians 6:11–18). These are spiritual weapons by which we can be “more than conquerors through Him who loved us” (Romans 8:37).