Brotherly love is the test of all claims to belong to the family of God, says John. The idea of brotherly love as a test of genuineness is found elsewhere in the New Testament.
One solemn reference concerns the Day of Judgement as it is described in Matthew 25 in the parable of the sheep and the goats. Jesus makes it clear in that passage that the presence or absence of brotherly love will indicate whether or not professors were true Christians. His words – ‘Truly, I say to you, as you did it to one of the least of these my brothers, you did it to me’ (Matt. 25:40) – ‘Truly, I say to you, as you did not do it to one of the least of these, you did not do it to me’ (Matt. 25:45) – reveal that brotherly love is the evidence of true Christianity. Actions express attitudes. If we don’t show brotherly love, John would say that we are not true Christians.
In addition, John makes clear that brotherly love is essential for Christian progress (vv. 10-11). We can only move on steadfastly towards heaven by walking in the light of brotherly love. Is John exaggerating when he says that brotherly love removes the possibility of stumbling? Let’s think about some sins of stumbling that happen when we don’t engage in brotherly love. Take sins of the tongue. If I don’t say wrong things about another Christian, I will not stumble in that way. Take sins of the heart such as jealousy of or despising another Christian’s position or achievements? If I stop such thoughts by appealing to Jesus for help, I won’t stumble in that way. Most sins in the Christian life or in a Christian church are caused by lack of brotherly love in one way or another.
Sometimes a person says that he has not made much progress in the Christian life. Maybe he should consider whether or not he is maintaining an attitude of heart that cannot be described as brotherly love. Because one thing is certain – the only people who are making progress are those who are marked by ongoing brotherly love that is fresh and new through a living union with Jesus Christ.