If you commit a crime and stand before God, (the just Judge), He must pronounce you guilty. Romans 6:23 says the penalty for doing wrong (sinning) is death. However, if someone died in your place (Christ did), your penalty would be paid (it is) and you’d be free (you are). But, you must accept or reject the substitute payment.
(Picture source: http://ronasays.blogspot.com/2010_09_12_archive.html).