14 We know that we have passed from death to life, because we love the brethren. He who does not love his brother abides in death. 15 Whoever hates his brother is a murderer, and you know that no murderer has eternal life abiding in him. (1 John 3:14–15, NKJV)
The apostle John had just described the children of God as those who “practice righteousness,” making an application that loving one’s brother is the epitome of this righteous conduct (1 Jno. 3:10). The brethren of whom he speaks are fellow Christians. One’s passage from spiritual death into spiritual life is assured by practicing love toward fellow Christians. As Cain, who murdered his brother Abel, the Christian who hates a fellow Christian is a murderer. A heart filled with hate has death abiding in it; not eternal life. To detest and disregard our brethren assures that we “abide in death” and will not pass into eternal life. (Oh yes, a Christian can sin and lose his soul!) This is a powerful motive to love each other as He has love us, and by obeying the Lord, abide in His love (Jno. 13:34-35; 15:10).