(A Course in Miracles)
We all make mistakes. We all step away from God's Path but mostly it is a missed step here or there and we immediately recognize that something is off. Sometimes we experience anger rising in us; on occasions we spot arrogance; there are times when we are careless and accidentally hurt our friends' feelings. But all that is not a big deal; we apologize afterwards and promise to do better next time around. Every missed step in fact makes the Way shine even brighter afterwards.
Sin is the deliberate looking in the wrong direction. It is the conscious journey away from God. Lust can blind us; greed and fear can lead us away from the Tao. Yet, while a seasoned spiritual traveler will experience these emotions just like everyone else, we physically just can't follow through. Walking away from God simply hurts too much. After you have experienced high energy vibrations you mind going back to the lower ones.
A spiritual traveler sees what is. It may be a vision at first, it may be foggy at times. Yet, 'what is' suddenly dawns on us in contrast to what 'appears to be'. There may still be a voice babbling in the back, there maybe a missed step along the Way but we can no longer be fooled by the stuff that appears to be. We reach the point when the eye sees what the heart knows and at that blessed moment we are Home.