The key is that we love each other.
Here the word of God shows us many things, such as: love is what distinguishes those who are children of God.
When we love others we are reproducing the life of Christ in us, this is for us like a kind of letter of introduction as Paul mentions.
Loving is not a simple feeling, loving is synonymous with action, just as God has shown us through the sacrifice of Jesus Christ, we also somehow demonstrate love in the way we treat other people, in the attitudes and words we use to refer to others.
I give you a new commandment: that you love one another; as I have loved you, that you also love one another.
John 13:34