Q: What is sin?
A: Sin is any want of conformity unto, or transgression of, the law of God.
1 John 3:4: Everyone who makes a practice of sinning also practices lawlessness; sin is lawlessness.
I could try to expand upon this idea some more to truly show the gravity of sin and the offense that sin is to a perfectly holy and just God, but why try to say something that others have said better? I heard this quote from John Piper a few years ago, and it is by far one of the most startling statements of what sin truly is:
"What is sin? The glory of God not honored.
The holiness of God not reverenced.
The greatness of God not admired.
The power of God not praised.
The truth of God not sought.
The wisdom of God not esteemed.
The beauty of God not treasured.
The goodness of God not savored.
The faithfulness of God not trusted.
The commandments of God not obeyed.
The justice of God not respected.
The wrath of God not feared.
The grace of God not cherished.
The presence of God not prized.
The person of God not loved.
That is sin."
You can hear his entire sermon, "What is Sin? The Root of All Sinning" here: