For this is the message which you have heard from the beginning, that we should love one another; not as Cain, who was of the evil one and slew his brother. And for what reason did he slay him? Because his deeds were evil, and his brother's were righteous.
From the beginning the message of the gospel was one of love. Jesus said, "By this all men will know that you are My disciples, if you have love for one another" (John 13:35).
If we are in Christ then when our sins are revealed through the Holy Spirit our hearts turn toward God and repentance. The flesh nature, like that revealed in Cain, leads people to blame others for mistakes, point the finger at people with “worse” sins, and even attack the righteous in an attempt to assuage their own guilty conscience. Cain was envious that his own offering was not accepted but his brother's was. We must guard ourselves from envy of those who seem favored by the Lord and strive to love all our Christian brothers and sisters (even the ones that within our flesh we find annoying or uncomfortable to be around). Just as Cain resented Abel's righteousness, so the world will resent our righteousness.