Genesis 4:3,4 -- And in the process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the Lord. And Abel, he also brought of the firstlings of his flock and of the fat thereof.
Cains offering was rejected because he offered the fruit of his own labors while abel offered a blood sacrifice that prophesied of the coming of Christ. This led to a rage in Cain that caused him to slay his brother.
The cross is the basis of our acceptance to God and remains the primary point of conflict between Christianity and every other religion, which are all based on man's acceptance through works.
1 Corinthians 1:18 -- The preaching of the cross is, I know, nonsense to those who are involved in this dying world, but to us who are being saved from that death it is nothing less than the power of God.
QUESTIONS TO ASK MYSELF:
Am I a "born again" person?
Who am I witnessing the power of God through the Cross of Jesus Christ to today?