"And yet it's not what WE are that is pleasing to God but rather the goodness of Him living and moving within us that is pleasing to Him.

Abel wanting to be washed clean brought a sin offering to God from his flock, while his brother Cain brought a tithe offering of the fruit from his gardens (Genesis 4:1-5). In essence, Cain was concealing his “dirt” rather than taking it to the Lord as a symbol of faith in the coming Messiah.

The Bible says that God had no respect for Cain or his offering.

When we come to God knowing who we are in His sight, we show faith in the atonement of Christ, becoming that sweet aroma that He calls us to be--trading pride for humility and shame for incomparable grace."

taken from: http://time-warp-wife.blogspot.com/2011/06/unto-god-sweet-savor-of-christ.html