12 [And] not be like Cain who [took his nature and got his motivation] from the evil one and slew his brother. And why did he slay him? Because his deeds (activities, works) were wicked and malicious and his brother's were righteous-virtuous.
13 Do not be surprised and wonder, brethren, that the world detests and pursues you with hatred.
14 We know that we have passed over out of the death into the Life by the fact that we love the brethren, [our fellow Christians]. He who does not love abides-remains, is held and kept continually-in [spiritual] death.
15 Any one who (abominates, detests) hates his brother [in Christ] is [at heart] a murderer, and you know that no murderer has eternal life abiding (persevering) within him. (1 John 3:11-15).
As the knowledge of God is tested by conduct (1:5-2:11), so being "born of God" (2:29) is tested by righteous action and love of fellow believers. The command to love fellow believers was first introduced in 2:9-11 as a test of whether one was walking in light, i.e., had true knowledge. Here it is the sum of the new life in God. In the former instance it was leveled as a charge against the heretics. Here it is addressed to the community of faith for encouragement and admonition. Most likely the disregard for love by the heretics had caused a lessening of an emphasis on love within the Christian community. The author presents the case for love first by the negative example of Cain (vv. 12-15), contrasted with the positive example of Jesus (v. 16).
11 The admonition that "we should love one another" is highlighted by the return to the critical formula. "This is the message you heard from the beginning," an almost identical reminiscence of 1:1, 5. The command to "love one another" has its origin in the nature of God. The entire goal and aim of the Gospel is to create and strengthen love (1 Tim 1:5).
12 The mention of Cain points back to 3:8 and reminds us that hatred is also from the beginning. The choice between the children of God and the children of the devil, between hatred and love or life and death, stems from the earliest moment of human existence. It also points to Jn 8:37-47, where some Jewish opponents of Jesus had exhibited the same kind of hatred toward Jesus that Cain expressed toward Abel (Gen 4:1-8). There Jesus labels his enemies as people who "belong to your father, the devil ... [who was] a murderer from the beginning" (Jn 8:42).
The sequence of thought in this section is significant. It is not that Cain by murdering his brother became the child of the devil; but, being a child of the devil, his actions were evil and culminated in the murder of his brother. The reason why he "murdered" (lit., "butchered") him was that his brother's acts were righteous. Righteousness draws hatred from the devil and hatred from the children of the devil. Darkness cannot tolerate light; immorality, morality; hatred, love; or greed, sacrifice.
13 The hatred of the "world" for the community of faith must not surprise the believers. The author does not say that the world always hates believers; it did not always hate Jesus. But whenever the community of faith acts so as to expose the greed, avarice, hatred, and wickedness of the world, it must expect rejection; and if it should go so far as to interfere with its evil practices, as Jesus did in the temple, it may expect suffering and brutal death (cf. Jn 15:18-19, 25; 17:14).
14 John looks back to v.10 and answers the question: How do we know those who have been "born of God"? or, Who are the children of God? "We know that we have passed from death to life, because we love our brothers" (cf. Jn 5:24). This conviction is not based on self-judgment or self-justification but on the certainty that love is the basis for life in the believing community. This is a self-test by which each person can examine himself or herself. Love will not, of course, cause one's passage to spiritual life but will give evidence of it. Conversely, to be unable to love means that a person is without spiritual life and remains in death.
15 Here John links hatred with murder (cf. Matt 5:21-22). In the heart there is no difference; to hate is to despise, and murder is the fulfillment of that attitude. Cain, by murdering his brother, was cut off from the covenant community. So no murderer is within the community, nor anyone who "hates his brother." Such a person has no life of God and no fellowship with the faithful.
[NIV BIBLE COMMENTARY Volume 2: New Testament].
JESUS IS LORD.