An excerpt:

In order to know what "sin" is, we need to understand why God made us, and we've seen that God made us for the absolute highest purpose...to "image" Him...to reflect His Person and Glory to all of creation.  When we fail to "image" God...when we fail to reflect His Person and Glory to all of creation...we sin.  In Romans 3:23, Paul says that all people have sinned.  How does he know this?  He knows this because all people have "fallen short" of God's glory.  Based on our understanding of Genesis 1 and God's design for us, we could say that all people have sinned because they have failed to "image" God...they have failed to reflect His glory. 

Sometimes you hear people say, "sin is missing the mark".  What "mark" are we missing?  We often think of the mark as "perfection".  In other words, God expects us to be perfect and when we fail to be perfect, we miss the mark and sin.  Is this what Paul means?  I don't think it is.  The "mark" we are missing is the ultimate mark: the glory of God.  As I've said before, when we fail to reflect the glory of God...when we fail to "image" him...we sin...we miss the mark...we fail to function as He designed us to function.

You see, God didn't form a committee to come up with a bunch of "laws" and then say, "if you can keep this list of rules...then you'll be perfect".  The Law of God is designed to do one thing: point us to His person (& yes, ultimately to Christ).  A human being who lives according to God's law would function the way God designed him/her to function...they would image God.  Of course, only one person ever did that...and that was the second Adam...Jesus Christ.  Jesus was the perfect reflection of God's glory.  "The Word became flesh and made his dwelling among us. We have seen his glory, the glory of the One and Only, who came from the Father, full of grace and truth." This brings me full circle to a concise definition of sin: "Sin is lying to others (through our words, attitudes, actions, etc.) about who God is."