(If there are awards for longest time between posts in a blog series, I would be a serious contender.)
In one sense all people are children of God. Paul affirmed this in his sermon to the people of Athens, who were idol worshipers. He told them that God made us all so that we would seek him. He then corrected their understanding of God by pointing out that as offspring of God we should understand that God can’t be a stone that we ourselves make, “‘In him we live and move and have our being’; as even some of your own poets have said, ‘For we are indeed his offspring.’ Being then God’s offspring, we ought not to think that the divine being is like gold or silver or stone, an image formed by the art and imagination of man” (Acts 17:28-29).
All people are children of God in that they are created by God and made in the image of God.
However, the Injil also makes clear that those who believe in God through Jesus are adopted as children in a special sense. “But to all who did receive [Jesus], who believed in his name, he gave the right to become children of God” (John 1:12).
Making us sons and daughters is why God sent Jesus Christ. “But when the fullness of time had come, God sent forth his Son, born of woman, born under the law, to redeem those who were under the law so that we might receive adoption as sons. And because you are sons, God has sent the Spirit of his Son into our hearts, crying, ‘Abba! Father!’ So you are no longer a slave, but a son, and if a son, then an heir through God” (Galatians 4:4-7). Through Jesus Christ God has adopted believers into his family so that it is right and fitting to not only call him “Lord” or “God Over All” but also, “Father” and even “Daddy” (“Abba” is Aramaic [the language of Christ] and was less formal).
As sons and daughters of God adopted into his family, we enjoy all the benefits that we would expect for children of the King. In adoption we see God’s great love. “See what kind of love the Father has given to us, that we should be called children of God; and so we are” (1 John 3:1).
Children of God can be at peace because they know that God their Father will take care of all their needs (Matthew 6:25-33). They can be at peace because they know God their Father gives them good gifts (Matthew 7:7-11). And ultimately they can have peace because as sons and daughters they are also heirs of God. All that is his becomes theirs. “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” (Romans 8:16-17).
Part 1: Adam Is the Son of God
Part 2: The People of God (Israel) Are the Son of God
Part 3: The Son of David Is the Son of God
Part 4: Jesus Christ Is the Son of God
Part 5: Jesus Christ Is God’s Unique Son
Part 6: Believers Are Sons and Daughters of God

Dustin, I just came back to re-read some of these posts on the son of God. These are helpful – thank you. Can I suggest a related topic? – Jesus also calls himself the “son of man.” I would love to see this discussed in a similarly clear and comprehensive manner.
thanks!
Great idea Matt. When you write the posts I’ll gladly put them on the blog.