(As always a draft).
The key word that I will try to portray is a model. But first some definitions when dealing with the Trinity. It is necessary to point out the difference between the essential Trinity and the economical Trinity. The essential Trinity is a matter of the essence of the Triune God for His eternal existence; the economical Trinity is a matter of God's arrangement for His operation in His move to accomplish His eternal purpose.
The essential or the ontological Trinity refers to the essence of the Triune God for His existence. In His essence, God is one, the one unique God (Isa. 45:18b; 1 Cor. 8:6a). In the essential Trinity, the Father, the Son, and the Spirit coexist and coinhere at the same time and in the same way with no succession. There is no first, second, or third.
Essentially, God is one, but economically He is three—the Father, the Son, and the Spirit (Matt. 28:19; 2 Cor. 13:14). In God's plan, God's administrative arrangement, God's economy, the Father takes the first step, the Son takes the second step, and the Spirit takes the third step. The Father purposed (Eph. 1:4-6), the Son accomplished (vv. 7-12), and the Spirit applies what the Son accomplished according to the Father's purpose (vv. 13-14). This is a successive procedure or a succession in God's
economy to carry out His eternal purpose.
Karl Rahner, in his book, The Trinity, postulates:
There is only one outward activity of God, exerted and
possessed as one and the same by Father, Son, and Spirit,
according to the peculiar way in which each of them possesses the Godhead…
The activity which is common to all three persons and
appropriated only to one is (as with the divine essence)
possessed by each of the three persons in his own proper
way. The threefold way of subsisting of this activity (considered principiative) is as intrinsic and necessary for its
existence as it is necessary and essential for the divine
essence to subsist as threefold.
God's eternal purpose is to work Himself into us and make us the same as His Firstborn Son. The Firstborn Son of God is the standard model for God's economy. This standard model has both divinity and humanity. Originally, He was the Only Begotten Son of God. As the Only Begotten Son of God, He was the embodiment of God, for all that is in the Godhead was embodied in Him (Col. 2:9). He was also the expression of God (Heb. 1:3). As the embodiment and expression of God, He became incarnated, bringing divinity into humanity and joining humanity to divinity. But before the incarnation, divinity had never been joined to humanity. Since the day of His incarnation, there was in this universe a wonderful Person who was both human and divine.
Your question touches on the fundamental question of why there is a biblical revelation of the economic Trinity.
With the above background, we can see the Triune God's purpose is to work Himself into us and make us the same as His Firstborn Son (Rom. 8:29). The Firstborn Son of God is the standard model for God's economy. This standard model has both divinity and humanity.
Originally, He was the Only Begotten Son of God. As the Only Begotten Son of God, He was the embodiment of God, for all that is in the Godhead was embodied in Him (Col. 2:9). He was also the expression of God (Heb. 1:3). As the embodiment and expression of God, He became incarnated, bringing divinity into humanity and joining humanity to divinity. But before the incarnation, divinity had never been joined to humanity. Since the day of His incarnation, there was in this universe a wonderful Person who was both human and divine.
As the Only Begotten Son, Christ was already in glory. That did not require glorification. However, once He was on earth in His humanity, He needed to be glorified (John 17:1, Luke 24:26). Since He was already in glory, why did He still need to be glorified? Although He was in glory as the Only Begotten Son, since He had put on humanity, the human part of His being had to be glorified, to be processed into glory. In His resurrection Christ entered into His glorification (Luke 24:26).
Without knowing Christ, in His humanity, sets before his disciples, a model of One who partakes of the Father's divine nature, then Peter would never wrote that we will be "partakers of the divine nature". (2 Pet. 1:4).
In His divinity as the only begotten Son of God, Christ had authority over all. However, in His humanity as the Son of Man to be the King of the heavenly kingdom, all authority in heaven and on earth was given to Him after His resurrection. Christ lived as One who is under authority (seen in His contact with the centurion's sick daughter - the centurion who lived under authority saw and recognize Christ as One who does also). This became a model for the church life as the kingdom of God for the believers' exercise and equipping in the divine authority—Rom. 14:17; Rev. 1:9; Acts 14:22; Matt. 18:18.
As God, before His incarnation, the Lord Jesus was the Lord already. But after His incarnation, this very God became a man. God was the Lord, but man was not. Yet the Lord became a man, and this man was crucified on the cross, buried, and resurrected from the dead. It was at this time that the man Jesus became the Lord. For God to be the Lord, there is no need of any kind of inauguration. But for a small man from a lowly town in a despised country to be made the Lord requires a real inauguration. He was not made the Lord as God. As God He was Lord already. Now this man from Nazareth was inaugurated to be the Lord of all.
Peter, preaching on the day of Pentecost, said, "Therefore let all the house of Israel know assuredly, that God hath made that same Jesus, whom ye have crucified, both Lord and Christ." Peter not only preached Christ as the Savior, but also as Lord of all. He never realized that Jesus was so great before the ascension. By the ascension he realized that the very man Jesus, whom he had been following for three and a half years, had been inaugurated to be Lord of all!
In other words, the extent to which we understand and appreciate the economy of the mystery is the extent to which we will understand, appreciate, and experience. Our experience of the economic Trinity reveals the eternal will, the intentionality of the immanent Trinity, that we can partake further and deeper of the organic dispensing that He immanently is and through which He economically flows.
The Triune God is revealed economically in the New Testament to implicitly and subjectively communicate
the axiomatic principles of His being, thereby convincing
the believers of the reality of the possibility of fellowship,
the communion of communication, within the sphere
of the mingled, organic identity of God and man, first in
the person and work of the incarnated Son and then in
the enlarged sphere of the many sons of God.
because, axiomatically, He is both organic and He exists
as an eternal dispensing (Revelation 22:1).
