Sin is disobedience to God's commandments 1 John 3:4, Leviticus 5:17. Any disobedience to what God says is sin. Period.
God created angels. One angel, Lucifer, was the first of God's creation to sin and rebel against God 1 John 3:8. Lucifer, upon his rebellion, became the Devil and Satan. He convinced other angels to follow him in disobedience to God Matthew 25:41, Revelation 12:9. Sin started with the devil 1 John 3:8.
The Devil persuaded Adam and Eve, in the garden of Eden, to disobey God. They disobeyed by eating of the tree of the knowledge of good and evil Genesis 3:1-6, which God told them not to do.
To this day, the Devil continues to deceive. He is the mastermind behind all evil, and he rules over the sinful world 2 Corinthians 4:3-4, 1 John 5:18-19.
Today's Action:
Consider a recent instance whre you felt tempted to act contrary to God's commands. Reflect on the consequences of disobedience, not just in earthly terms but spiritually. What steps can you take to fortify your resolve against such temptations in the future?