Westminster Larger Catechism
Q74: What is adoption? A74: Adoption is an act of the free grace of God, in and for his only Son Jesus Christ, whereby all those that are justified are received into the number of his children, have his name put upon them, the Spirit of his Son given to them, are under his fatherly care and dispensations, admitted to all the liberties and privileges of the sons of God, made heirs of all the promises, and fellow heirs with Christ in glory.
Q33: Why is He called God's "only begotten Son," since we also are the children of God? A33: Because Christ alone is the eternal, natural Son of God, but we are the children of God by adoption, through grace, for His sake.
Q34: Why do you call Him "our Lord"? A34: Because not with silver or gold, but with His precious blood, He has redeemed and purchased us, body and soul, from sin and from all the power of the devil, to be His own.
On a side note, Pastor Lanning is my homiletics instructor as well as my pastor. As Professor Lanning, I do not think that he would allow me to get away with this quote, but he pulled it off quite well:
"Mr. Rogers taught us that all grows together because we are all one piece... that is good human biology as well as good Christian Theology".
It is worth spending the 40 minutes in meditation on our adoption as sons!