That he was originally created a holy and perfect being, but through pride and ambition rebelled against God (Isaiah 14:13,14; Ezekeil 28:13-17; I Timothy 3:6), thus becoming: Utterly depraved in character (John 8:44).
The great adversary of God and His people (Matthew 4:1-11; Revelation 12:10).
Leader of all other evil angels and spirits (Matthew 12:24-26; 25:41).
The deceiver and god of this present world (II Corinthians 4:4; Revelation 12:9).
That his powers are supernaturally great (Luke 4:6; II Corinthians 4:3; Ephesians 6:12; II Thessalonians 2:9), but strictly limited by the permissive will of God (Job 1:12; Luke 22:31,32).
That he was defeated and judged at the cross, and therefore his final doom is certain (John 12:31,32; 16:11; Hebrews 2:14; Revelation 20:10).
That we are to resist and overcome him only in the armor of God and by the blood of the Lamb (Ephesians 6:1-18; Revelation 12:11; Zechariah 3:2).