The wonder of Christmas is that Jesus is the son of God; God made families to reflect himself. Genesis I tells us that we were made male and female in his image. Then Jesus became human, being born into a human family. Christmas is a family time, and very rightly.
Of course, families are not just a wonderful means of relationship; children produce more children. We become children of God through Christ, the son of God (1 Jn 3:2). We are God's family, so seeing what it means for Jesus to be Son of God helps us to reflect that.
Son derives from the Father. We derive our being from our parents. Thus Jesus is fully God as son of His Father and fully human as son of Mary. It is only because He has this dual nature that He is able to be our sacrifice.
In the same way, our new spiritual life derives from God. Just as the Spirit came on Mary to enable the incarnation, so we receive new birth by the Spirit.
But there is one big difference from a human birth, which reflects the divine nature of Jesus; He receives in an eternal giving. So once we have received Christ we must then constantly be filled!
Son is like the Father. We are naturally like our fathers, and Jesus shows what God is like. The word "son" in the Bible often denotes likeness eg Acts 4:36
When we become Christian, we are naturally like our heavenly Father. However we can also choose to deliberately imitate him, becoming more like him. That is good, as it is easier to live with somebody you are like. Remember that we will be with him forever, so get like him now!
Son is in harmony with the Father. Because of the interaction between Father and Son in the Trinity, they are equal, and always in agreement. Jesus says, "I always do the will of my Father" (Jn 8:29).
Likewise, as God's children we should be in harmony with God (1 Jn 3:3). It should be our aim to both will and do what He wants. In the Trinity, the harmonious relation is facilitated by the Spirit, who also then works to conform us to God.
God sent His Son at Christmas to serve him and to build His Church, and as children of God, he sends us likewise.
Sermon by Prof David T Williams