Q. 38. What is adoption?
A. Adoption is an act of God’s free grace, whereby we are received into the number, and have a right to all the privileges of the sons of God.
See what kind of love the Father has given to us, that we should be called children of God; and so we are. The reason why the world does not know us is that it did not know him. 1 John 3:1
But to all who did receive him, who believed in his name, he gave the right to become children of God, John 1:12
The Spirit himself bears witness with our spirit that we are children of God, and if children, then heirs—heirs of God and fellow heirs with Christ, provided we suffer with him in order that we may also be glorified with him. Romans 8:16-17