The Mark of Cain
1John 3:11,12 This is the message you heard from the beginning: We should love one another. Do not be like Cain, who belonged to the evil one and murdered his brother. And why did he murder him? Because his own actions were evil and his brother’s were righteous.
Prior to the murder, the only thing recorded of Cain is the following:
Gen 4:3-5 In the course of time Cain brought some of the fruits of the soil as an offering to the LORD. But Abel brought fat portions from some of the firstborn of his flock. The LORD looked with favor on Abel and his offering, but on Cain and his offering he did not look with favor. So Cain was very angry, and his face was downcast.
Apparently there was something evil about Cain's sacrifice. It says in Heb 11:4,5 "By faith Abel offered God a better sacrifice than Cain did. By faith he was commended as a righteous man, when God spoke well of his offerings."
While under the Law of Moses grain offerings were accepted, they were contingent upon blood offerings to atone for sin. The modern equivalent would be to feign friendship with God apart from the blood offering made by Jesus Christ. That is to enter God's house not through the door but through some other way.
Cain was involved in a religious activity which outwardly was to honor God, but in fact he was doing evil. So also both in Jesus' time and today, those most hostile against the children of God are those who are zealously involved in false worship. And those who are of the evil one, rather than dealing with their own guilt will characteristically lash out against those who are behaving properly.