John in this section of his letter is writing about the importance of brotherly love and in verse 8 he says that it should always be a fresh experience. He links this fresh experience of brotherly love to the experience of Jesus as well as to the experience of his people when he says that it is true in Jesus as well as in them. This newness is therefore an expression of the reality of union with Christ.
John says more about brotherly love when he says that it is true as well as new. The word ‘true’ has different meanings: it can refer to what is right as against what is wrong; it can refer to what is genuine as against what is pretence; and it can refer to what is lasting as against what is temporary. Sometimes the word has one of those meanings, at other times it includes them all. John here wants us to understand the word as expressing all of them. True brotherly love is always right and genuine because that is how Jesus reveals true brotherly love.
It could be that John is referring to the way that Jesus revealed ‘brotherly love’ when he was here on earth – such as his forgiveness of the sins of his disciples, his patience with their ignorance of and indifference to his teaching, his self-sacrifice on their behalf on the cross, and his desire to meet them on the morning of his resurrection.
Yet I think John, in using the present tense, is also asking his readers to think about the current brotherly love of Jesus: although he is in heaven at a spatial distance from them, he still provides for their needs, forgives their sins, shows his patience, and desires intensely the day when they shall be in his presence forever. So we can say that the current love of Jesus, as well as the love he showed when he was here on earth, is a role model for his disciples to imitate every day. While this love will not be in them in the perfection which it is in Jesus, it will be in them to a degree. In other words, brotherly love reveals union with Jesus in heaven.