Commentators and theologians struggle with a perceived tension in John's Christology. On the one hand, John has high Christology, stressing the full divinity of Christ. On the other hand, there's the apparent "subordination" in John. Recently, I was reading an article by Richard Bauckham. After exegeting some passages in the Fourth Gospel that demonstrate the full divinity of Christ, Bauckham goes on to observe:
In the much-debated statement "the Father is greater than I" (14:28) the reference is probably to the Son's dependence on the Father's giving, not to the Son's obedience to the Father, which is not relevant to the context. The use of the term "subordination", which implies a hierarchy of rank, may therefore not be very helpful. The Johannine account implies not that the Son ranks below the Father, but that the Son owes everything to the Father. Since everything is given, there is both asymmetry (14:28) and complete commonality (16:15; 17:10). “The Trinity and the Gospel of John,” in The Essential Trinity: New Testament Foundations and Practical Relevance, ed. by Brandon D. Crowe and Carl R. Trueman (London: Inter-Varsity Press [Apollos] 2016), 110.
We should be careful to follow the outlines of the way the Gospel actually depicts the Father-Son relationship, which does not conform in every respect to the relationship of human fathers and sons in the ancient world. A key difference is the fact that both Father and Son are eternal. In a restricted sense the Son's position resembles that of a son who has inherited his father's status and estate at the latter's death. Ibid. 110n55.
The theological metaphor of the messiah as David's heir and God's heir is a common motif in OT prophecy, viz. Pss 2; 72; 89; 110; 132; Isa 9; Dan 7.
This metaphor is picked up in the NT, viz. Mt 21:33-46, Eph 1; Phil 2:9-11; Col 1:15-20; Heb 1, Revelation.
Let's begin with the human analogue. A grown son is his father's natural equal. They are both adult men, with comparable natural abilities.
In reference to royal succession, a prince is "subordinate" to the king, his father's legal inferior, unless and until the son assumes the throne. At that point he may be his father's legal peer (in a cogency), or his father's legal superior (if the king abdicates the throne in favor of his son).
The reason for royal succession is human mortality. Sometimes the prince inherits his father's throne when the king dies in office. Sometimes the king abdicates the throne to ensure a peaceful transfer of power. To insure that his designated heir will ascend to the throne–rather than a usurper. Sometimes the king makes his son coregent.
In this human legal context, if a son is an heir, then he receives something from his late father–and if he's the sole beneficiary, then he receives everything from his late father. All his royal prerogatives pass on to the son.
Another aspect of this metaphor is that only a loyal, faithful, obedient son is his father's chosen heir. A father, and especially a king, won't ordinarily leave his estate or his throne to a disloyal son.
It's easy to see how much of this framework carries over into OT messianism and NT Christology. We see an interplay between the Son's natural status as the Father's equal, and his legal status as the Father's temporary inferior. Yet as the rightful heir, he will take his place as the Father's legal peer.
We might also ask how much of this is literal and how much is anthropomorphic:
i) When the Son returns to the Father (Jn 17:5), this isn't just a resumption of the status quo ante, for he returns as the Incarnate Son. Something has changed. The Incarnation elevates the humanity of Christ. In addition, the Son's successful mission, and "reward," is conferred on the attached humanity.
ii) Clearly divine succession isn't dependent on the usual vicissitudes of human mortality. Just as we shouldn't think of the Father as an aging, failing monarch, it's a mistake to press the "subordination" of the Son. We need to be consistent. It's ad hoc to exempt the Father from the anthropomorphic connotations of the metaphor, but not the Son.
iii) I think God created a world with fathers and sons, kings, princes, and heirs, as preparation for redemption. That creates a reservoir of theological metaphors which the Bible taps into.
iv) I think there's a degree of divine accommodation, where Father and Son assume roles familiar to human social relations and social conventions. That makes the nature of redemption comprehensible to human readers.
On the one hand, it's not just a literary depiction. On the other hand, this is playacting for the benefit of the redeemed. The Trinitarian division of labor in the economy of salvation is real, but not because there's eternal, metaphysical subordination in the immanent Trinity. Rather, this is an example of divine condescension. God makes himself more accessible by coming down to our level, since we can't rise to his level.