A lot of our life in this world takes place in families (as I write this a family of crows is in the trees outside, conversing in their funny voices). We have concern for our families and try our best to help them. We feel responsibilities to our families and suffer when they suffer.
Sometimes we look like our parents or share physical characteristics, for better or worse. We inheirit hair color or height. More powerfully, we learn behaviors from our parents, and values. When I think of my mother I remember her car trunk full of pies one day. A bakery had donated them and she was taking the time and trouble to pass them on to someone who may need a pie! One of her values was taking time to help people.
In this passage John says, God loves us so much that we are now the children of God; what a wonderful statement. Family resemblance; John says, should be apparent. If we claim that God is our spiritual parent people should be able to tell who our parent is by our actions. We will see God, and then we will be like him (verse 2), so, having this hope, we learn behavior from God and, anticipating being like God we behave like God.
John then goes on to explain what behavior we should learn from God; righteousness and love. We will not be sinless but we should persist in emulating our Father. John brings this passage to a close with a very direct example of what God wants; love our fellow Christians. Today may we love those in our faith family .