Cain's offering of the goods from his harvest were rejected by God. God had deemed Cain's offering unacceptable because Cain did not present his offering with a pure heart.
The angered Cain was warned by God, "if you do not do what is right, sin is crouching at your door, it desires to have you, but you must rule over it."
God spoke directly to Cain and yet Cain did not have faith. How many of us would ignore God if He were to speak to us directly? Would we not obey and wouldn't any doubt about God be completely eliminated?
At this point in time, there are likely less than 100 people on earth and yet Cain proceeds to murder his brother. Did Cain not believe that God was omnipresent and did Cain believe that God would be deceived? And yet Cain tried to do just that.
So here is a man who has spoken with God and still does not have faith in God, or His power.
God has not spoken to man for centuries. We have His word in the Bible, but He has not shown himself to us physically or conversed with us directly. Our belief is based solely on faith.
I wonder just how arrogant Cain must have been. Much more arrogant than the worst atheists we have today. He saw and spoke to God and rejected God's way.