According to some commentaries that I have read, Cain was rejected by Hashem in Genesis 4 because he did not offer the first or choice of his field, but rather the dregs or at least standard fare. The way in which Hashem exiled Cain, however, does strikestrikes me as similar to the treatment of an idolater, which makes me wonder:
Was Cain an idolater (that is, did he actively worship an idol or indicate an express intent in doing so), or was his sin simply in being an ineffective monotheistic worshiper (e.g. his theology was right, but his praxis was wrong)?