What is sin?

A prideful heart.

Proverbs 21:4 An high look, and a proud heart, and the plowing of the wicked, is sin.

A thought of foolishness.

Proverbs 24:9 The thought of foolishness is sin: and the scorner is an abomination to men.

Transgression of the law.

1 John 3:4 ...for sin is the transgression of the law.

All unrighteousness.

1 John 5:17 All unrighteousness is sin: and there is a sin not unto death.

Sin is WHY Christ came into the world!

1 Timothy 1:15 This is a faithful saying, and worthy of all acceptation, that Christ Jesus came into the world to save sinners; of whom I am chief.