Paul is, in this piece, thinking of the significance of adoption in the Roman world. Adoption meant a new life, a cancelling out of all that had gone before – former family and all that entailed. It was a new identity Paul was referencing here and the person who became Christian belonged to a new family the family of God attested by the Spirit. In being adopted the person becomes heir and in Christ we are fellow heirs of God’s kingdom.
If you ever need to feel you belong, this is it. You are a child of God. Go out today and celebrate that fact.