David Kirk notes that ‘while the creation is a work of the whole Trinity…it also stands in a peculiar relation to the Son’ (citing Herman Bavinck, Reformed Dogmatics, 2:423).
In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God. All things were made through him, and without him was not any thing made that was made… (John 1:1-3 ESV)The Logos becomes flesh because he is the one through whom and for whom all things were made (Colossians 1:16). The world, made through the Son, is to be saved through the Son:
For God so loved the world, that he gave his only Son, that whoever believes in him should not perish but have eternal life. For God did not send his Son into the world to condemn the world, but in order that the world might be saved through him. (John 3:16-17 ESV)Mark Jones offers the following reasons (my headings and combining the last two reasons):
(1) The title "Son" for one person of the Trinity only
The Son of God is, by virtue of his title, more appropriately the Son of Man and the Son of a woman. In other words, it was not “fit” that in the Trinity there should be two persons who both bear the title of “Son,” which would have been the case had the Father become incarnate.
Turretin argued that the Holy Spirit, for example, could not be sent to be Mediator because “there would have been two sons, the second person by eternal generation and the third by an incarnation in time.”(2) The Son is the middle person within the Trinity
the Son, as the “middle person” bears the best resemblance of the work as Mediator. He comes between us and God.
Turretin argues that “he who is between the Father and the Holy Spirit should be Mediator between God and men.”(3) Adoption as sons of God is the aim of salvation
The Son is peculiarly fitted to be Mediator since, according to Thomas Goodwin, “the main end of his being Mediator,” that is, the adoption of his people into the family of God, is “made one of the greatest benefits of all others” (Eph. 1:5).(4) The offices of priest, prophet, and king are most apt for the Son
The Son is the most suitable person to convey this soteric blessing insofar that as a Son Christ conveys sonship upon his people by virtue of his union with them (Gal. 4:4-5).
Again, in similar fashion, Turretin argues that it was fitting that “he who was a Son by nature should make us adoptive sons by grace.” Besides Trinitarian reasons, soteric factors – i.e. the doctrine of adoption – explain why the Son should be Mediator.
Regarding the office of priest, it is the birth-right of the eldest Son in the family to be the priest. Therefore, to prove he was a Priest (Heb. 5), the author cites Psalm 2: “You are my Son, today I have begotten you.” As an intercessory priest the Son is uniquely able to approach the Father, which is a function grounded both in ontology (i.e. their natural subsistence) and economy (Christ’s work of mediation).[The last paragraph is an assertion rather than an argument. The argument may be in the first paragraph, given that the citation of Psalm 2 is more apt for kingship than priesthood. Priesthood and kingship are of course combined in Melchizedek, cf. Psalm 110.]
As a prophet, the Son is especially fit to be Mediator because he is the Word and Wisdom of the Father (Heb. 1:1; Jn. 1:18).
As a King, there is none so fit as the heir, “none so fit to have all Judgment and the Kingdom committed to him as God’s Son” (Goodwin).