In Jeremiah 2:13, God describes the sin of His people this way: “My people have committed two evils: they have forsaken me, the fountain of living waters, and hewed out cisterns for themselves, broken cisterns that can hold no water.” Sin is not only that we do wrong, but that we are wrong at the deepest levels of our existence. We are not just imperfect, but morally and spiritually insane. Ultimately, our problem with sin is a problem with with God.

Here’s a helpful definition of sin by pastor, John Piper:

What is sin?
It is 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.
—John Piper, Let The Nations Be Glad: The Supremacy of God in Missions, 206

As we grow in our Christian life we grow in our understanding of how great a problem the problem of sin really is. A right understanding of sin, of course, helps us better understand what it is that Jesus Christ did for us: “For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God” (2 Cor. 5:21).