Part Six: The Trinity and Adam's Sonship (read the other parts here)
We concluded Part Five by asking this question: In what sense was Adam a son of God? Have you ever considered why it is that God is three persons and not just one? (This question is relevant to our discussion of Adam's sonship. So stay with me.) I am one person. You are one person. So it just makes sense to me that God would be one person, but Scripture reveals God to be three persons not one. Here is our agency's doctrinal statement concerning the Trinity: "There is one true God, eternally existing in three equally divine persons: Father, Son, and Holy Spirit." Scripture teaches that God is one God eternally existing as three persons. So why is the one God three persons and not just one person?
Believe it or not, theologians have wrestled with this very question. Richard of St. Victor, a Scottish theologian of the 12th century, wrote an important philosophical work on the Trinity titled De Trinitate ("On the Trinity"). When wrestling with why it is that God is three persons and not just one, an answer to this difficult question struck him while meditating on 1 John 4:8 ("God is love") and 1 Corinthians 13, the great love chapter.
Here was Richard of St. Victor's profound insight: Since 1 Corinthians 13 teaches that love is never turned in upon itself but is always turned out upon other persons and 1 John 4:8 teaches that God is love, God could not be love if He were only one person. A god who existed all eternity past as one person would be a god who was eternally turned in upon himself. This god could not be love since, according to Scripture, love is always turned outward upon other persons. Therefore, he concluded, a God who is love must be more than one person.
Scripture teaches that God is a communion of persons. God has never been a solitary individual. The Triune God has always enjoyed perfect communion and joy as Father, Son, and Holy Spirit. For all of eternity the Father has loved the Son, the Son has loved the Father, and the Spirit has been the personal bond of that communion. The Father, Son, and Holy Spirit have eternally been and will forever be a communion of persons. Therefore, when considering God's creation of man, we must not think that God created him because He was lonely! God did not need to create anything, man included.
The Creation of Man
So if God did not need to create man, why did He do it? As Jonathan Edwards has written, "It is no argument of the emptiness or deficiency of a fountain, that it is inclined to overflow." The creation of man was the kind, gracious, intentional overflow of the triune God's joy and love in order to increase the circle of communion and make His glory known. Ultimately, as Kevin Vanhoozer writes, the gospel itself "concerns the triune God's self-communication for the purpose of enlarging the circle of communion. The gospel proclaims a new possibility, namely, that of becoming a 'communicant' in the life of God." God created and now redeems so that man might share in the communion of the Trinity to the praise of the glory of His grace (Ephesians 1:6).
You may be wondering, "What does all this discussion about the Trinity have to do with the creation of man? What does the Trinity have to do with Adam's sonship?" Well, after saying, "Let there be" each day of the creation week, what did God say when He paused to create man? He did not say, "Let there be man." No, God said, "Let us make man in our own image" (Genesis 1:26). When He created man, God's modus operandi changed. Something very personal transpired when God made man.
Adam and Eve, in contrast to everything else God had previously created, were made in the image of the Triune God, in the image of the God who is Father, Son, and Holy Spirit. This means that God did not create man to be a solitary being. Since God is a communion of persons (i.e., persons-in-community) and we were made in His triune image, it is clear that we were created to live in communion with other persons—with the Father, Son, and Holy Spirit and with other human beings. This is central to what it means to be created in the image of God.
Yes, being made in the image of God involves exercising dominion over the earth as those who are under the benevolent rule of God; but we are meant to exercise that dominion as those who are in joyful relationship with God (who is Father, Son, and Holy Spirit) and other human persons, as those who are persons-in-community with other persons. This was Adam's sonship. It was in this way that Adam was a son of God. Adam and Eve were created to exercise their dominion over the earth in joyful communion with God the Father and His eternal Son in the fellowship of the Spirit (2 Corinthians 13:14). 
 Dr. Douglas Kelly, in his Systematic Theology I lectures at Reformed Theological Seminary, addressed this very question. My comments here are based upon the portion of his lecture on the Trinity that addressed this issue.
 From John Piper's God's Passion for His Glory: Living the Vision of Jonathan Edwards, 156.
 The Drama of Doctrine: A Canonical, Linguistic Approach to Christian Theology, 53.
 "The Plural pronoun, 'Let us make man', suggests a God who is plural and communal. God creates through his word and now that word is addressed to himself. God is personal and he exists in community. What constitutes the image of God in man is a much-debated issue, but it seems that one element of what it means to be God's image bearers is this communal nature. The God who said 'Let us' makes us relational beings. We are people in community. He did not make us solitary. We are made male and female. We are made to exist in community and we are made for community with God. The Trinitarian community graciously extends its communal life" (Tim Chester, From Creation to New Creation, 42).
 "How important is sonship in biblical teaching? We can express its centrality abruptly but truly by saying: Our sonship to God is the apex of creation and the goal of redemption. God's first man was created as his image in order to be his son. Luke suggests that Adam was 'the son of God' in his genealogy of the Lord Jesus Christ (Lk. 3:38).To be a son, in the language of Genesis, was to be made in the image and likeness of one's father. So, when Seth was born to Adam and Eve, the event is recorded in these terms: 'When Adam had lived 130 years, he has a son in his own likeness, in his own image' (Gen. 5:3). Exactly the same phrase is used about the relationship between God and Adam. God made man in his image and likeness (Gen. 1:26-27; 5:1-2). To be a son, and to be the image and likeness of your father, are synonymous ideas. To put it another way, if we wish to understand what man was intended to be, we need to think of him as a son of God" (Sinclair Ferguson, Children of the Living God, 6-7).