In Genesis 4 Fr. Daniel Poovannathil explains the offerings made by Abel & Cain, and the repurcussions of their lifestyles in God's acceptance of the offerings. We pray to god for mercy with hearts filled with Pride, Jealousy and Anger that God cannot accept our prayers because he cannot remain oblivious of our thoughts and our intentions. Cain was proud, jealous and angry and for these sins God was not happy with him first and then his offerings, so would be the case with our sacrifices and our offerings lest we forgive and live a life according to the wishes of the Lord. This teaching can also be viewed on YouTube at https://www.youtube.com/watch?v=wEcZuYha3Wg