Part 5: The Question (read the other parts here)
As I have tried to demonstrate in parts 3 and 4 in this series, it really is not difficult to recognize the importance of adoption in Paul's thought. It clearly plays a central role in the outworking of the history of redemption. Adoption precedes human history (in God's pre-temporal decision to love us, Ephesians 1:4-5), shows up at climactic moments within redemptive-history (Romans 9:4; Galatians 4:4), and brings our salvation to its intended goal (Romans 8:23).
You may be wondering, though, why adoption is so important in the unfolding story of redemption. It is one thing to recognize its importance. It is another thing to understand why adoption is important. Therefore, I think it will be helpful to view the story-line of redemption (i.e., creation, fall, redemption, consummation) through the lens of the doctrine of adoption.
Here is the outline that will lead us through the story of redemption from the perspective of adoption over the next several posts:
- Adam's Sonship (Creation / Fall)
- Abraham's Promise (Redemption)
- Israel's Adoption (Redemption)
- Jesus' Mission (Redemption)
- The Spirit's Work (Consummation)
Adam's Sonship (Creation / Fall)
Sometime after God's pre-temporal decision to adopt us (Ephesians 1:5), He created the heavens and the earth and, on the sixth day of creation, made man in His own image (Genesis 1:1, 26-27). Creation week reached its climax when God formed the first man, Adam, from the dust of the ground. In his genealogy of Jesus, Luke begins with Jesus and works his way backward through time all the way to Adam: "Jesus, when he began his ministry, was about thirty years of age, being the son (as was supposed) of Joseph, the son of Heli..." (Luke 3:23). All throughout the remainder of the genealogy we see the phrase "the son of…"  Notice how the genealogy ends: "The son of Enos, the son of Seth, the son of Adam, the son of God" (Luke 3:38; emphasis mine). Surprisingly, Luke refers to Adam as the son of God!
Why did Luke do this? I think it is very important to see how Luke frames his genealogy of Jesus. It is immediately preceded by his account of Jesus' baptism where we read what God the Father spoke after Jesus came up out of the water: "and the Holy Spirit descended on him in bodily form, like a dove; and a voice came from heaven, "You are my beloved Son; with you I am well pleased" (Luke 3:22). Jesus is identified as God's beloved Son.
Jesus' genealogy is then immediately followed by the account of his temptation (Luke 4:1ff). Of the devil's three temptations, two begin with "If you are the Son of God" (Luke 4:3, 9; emphasis mine). On both sides of this genealogy where Luke identifies Adam as a son of God, we find Jesus identified as the Son of God. It seems to me that Luke intends that we see a connection between the sonship of Adam and the Sonship of Jesus.
It is helpful to see the framing of Luke's genealogy in contrast to Matthew's. Matthew opens his Gospel with Jesus' genealogy (Matthew 1:1-17). He then follows it with the account of Jesus' birth (Matthew 1:18-25). The baptism and temptation narratives, which are placed side-by-side in Matthew's Gospel, do not appear until the end of chapter three and the beginning of chapter four respectively.
The Gospel of Luke, unlike Matthew, places Jesus' genealogy in between his baptism and temptation narratives. Here's the sequence in Luke's Gospel: First, his baptism narrative ends by identifying Jesus as God's beloved Son (Luke 3:22). Second, his genealogy of Jesus begins with God's beloved Son, Jesus (Luke 3:23), and ends with Adam, God's disobedient son (Luke 3:38). Third, Luke then follows the genealogy with the temptation of God's beloved Son who succeeds where Adam failed (Luke 4:1-13). It seems that Luke structured these two chapters in this way so that we would see the successful Sonship of Jesus in contrast to the failed sonship of Adam. 
Before we go any further, let me add parenthetically that Luke is not saying, nor am I saying, that Adam was divine in any way whatsoever. There is only one divine Son, and he is the second person of the Trinity. So, that raises this question: In what sense was Adam a son of God? We will consider this in part 6.
 The Greek text only has "son" the first time: "the son of Joseph" (Luke 3:23). Even though it is absent throughout the remainder of the genealogy, it is implied in each case.
 Edmund Clowney writes: "Christ's testing came at the very outset of His ministry. It was the Holy Spirit who drove Christ into the desert: the Spirit of the Father who came upon Him at His baptism—the Spirit, therefore, of His Sonship. 'Thou art my beloved Son; in thee I am well pleased' (Lk. 3:22, KJV). Adam was tested that he might be confirmed in his sonship. Jesus was tested in sonship, too. He was tested as the Messianic Son who was also the only begotten and beloved Son of the Father: the divine Son in human flesh" (The Unfolding Mystery: Discovering Christ in the Old Testament, 28).