How important is the doctrine of uppercase Adoption (i.e. God adopting us)? If you are passionate about lowercase adoption (i.e. couples adopting children), this is an important question to answer. The importance of uppercase Adoption establishes the importance of lowercase adoption. Here's what J. I. Packer has to say about the question:
"You sum up the whole of New Testament teaching in a single phrase, if you speak of it as a revelation of the Fatherhood of the holy Creator. In the same way, you sum up the whole of New Testament religion if you describe it as the knowledge of God as one's holy Father. If you want to judge how well a person understands Christianity, find out how much he makes of the thought of being God's child, and having God as his Father. If this is not the thought that prompts and controls his worship and prayers and his whole outlook on life, it means that he does not understand Christianity very well at all" (J.I. Packer, Knowing God, 182).
[...] Adoption: No Better Way to Understand Christianity If you want to judge how well a person understands Christianity, find out how much he makes of the thought of being God’s child, and having God as his Father. —J. I. Packer [...]