Why was it requisite that the Mediator should be God?
It was requisite that the Mediator should be God, that he might sustain and keep the human nature from sinking under the infinite wrath of God, and the power of death;1 give worth and efficacy to his sufferings, obedience, and intercession;2 and to satisfy God's justice,3 procure his favor,4 purchase a peculiar people,5 give his Spirit to them,6 conquer all their enemies,7 and bring them to everlasting salvation.8
Scripture proofs
The Westminster Assembly's proof texts (KJV). The bracketed numbers in the answer above mark the clause each set of references supports; expand a row to read the verses.
1 Acts 2:24–25; Rom. 1:4; Rom. 4:25; Heb. 9:14
Acts 2:24 Whom God hath raised up, having loosed the pains of death: because it was not possible that he should be holden of it.
Acts 2:25 For David speaketh concerning him, I foresaw the Lord always before my face, for he is on my right hand, that I should not be moved:
And declared to be the Son of God with power, according to the spirit of holiness, by the resurrection from the dead:
Who was delivered for our offences, and was raised again for our justification.
How much more shall the blood of Christ, who through the eternal Spirit offered himself without spot to God, purge your conscience from dead works to serve the living God?
2 Acts 20:28; Heb. 7:25–28; Heb. 9:14
Take heed therefore unto yourselves, and to all the flock, over the which the Holy Ghost hath made you overseers, to feed the church of God, which he hath purchased with his own blood.
Heb. 7:25 Wherefore he is able also to save them to the uttermost that come unto God by him, seeing he ever liveth to make intercession for them.
Heb. 7:26 For such an high priest became us, who is holy, harmless, undefiled, separate from sinners, and made higher than the heavens;
Heb. 7:27 Who needeth not daily, as those high priests, to offer up sacrifice, first for his own sins, and then for the people’s: for this he did once, when he offered up himself.
Heb. 7:28 For the law maketh men high priests which have infirmity; but the word of the oath, which was since the law, maketh the Son, who is consecrated for evermore.
How much more shall the blood of Christ, who through the eternal Spirit offered himself without spot to God, purge your conscience from dead works to serve the living God?
3 Rom. 3:24–26
Rom. 3:24 Being justified freely by his grace through the redemption that is in Christ Jesus:
Rom. 3:25 Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God;
Rom. 3:26 To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus.
4 Eph. 1:6; Matt. 3:17
To the praise of the glory of his grace, wherein he hath made us accepted in the beloved.
And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased.
5 Titus 2:13–14
Titus 2:13 Looking for that blessed hope, and the glorious appearing of the great God and our Saviour Jesus Christ;
Titus 2:14 Who gave himself for us, that he might redeem us from all iniquity, and purify unto himself a peculiar people, zealous of good works.
6 Gal. 4:6
And because ye are sons, God hath sent forth the Spirit of his Son into your hearts, crying, Abba, Father.
7 Luke 1:68–69, 71, 74
Luke 1:68 Blessed be the Lord God of Israel; for he hath visited and redeemed his people,
Luke 1:69 And hath raised up an horn of salvation for us in the house of his servant David;
Luke 1:71 That we should be saved from our enemies, and from the hand of all that hate us;
Luke 1:74 That he would grant unto us, that we being delivered out of the hand of our enemies might serve him without fear,
8 Heb. 5:8–9; Heb. 9:11–15
Heb. 5:8 Though he were a Son, yet learned he obedience by the things which he suffered;
Heb. 5:9 And being made perfect, he became the author of eternal salvation unto all them that obey him;
Heb. 9:11 But Christ being come an high priest of good things to come, by a greater and more perfect tabernacle, not made with hands, that is to say, not of this building;
Heb. 9:12 Neither by the blood of goats and calves, but by his own blood he entered in once into the holy place, having obtained eternal redemption for us.
Heb. 9:13 For if the blood of bulls and of goats, and the ashes of an heifer sprinkling the unclean, sanctifieth to the purifying of the flesh:
Heb. 9:14 How much more shall the blood of Christ, who through the eternal Spirit offered himself without spot to God, purge your conscience from dead works to serve the living God?
Heb. 9:15 And for this cause he is the mediator of the new testament, that by means of death, for the redemption of the transgressions that were under the first testament, they which are called might receive the promise of eternal inheritance.
This doctrine across the Standards
Where the Confession and the Westminster Shorter Catechism treat the same matter, so the question can be studied alongside its parallel statements.
In the Westminster Confession
VIII.2 The Son of God, the second person in the Trinity, being very and eternal God, of one substance, …
The Son of God, the second person in the Trinity, being very and eternal God, of one substance, and equal with the Father, did, when the fulness of time was come, take upon him man's nature, with all the essential properties and common infirmities thereof, yet without sin: being conceived by the power of the Holy Ghost in the womb of the Virgin Mary, of her substance. So that two whole, perfect, and distinct natures, the Godhead and the manhood, were inseparably joined together in one person, without conversion, composition, or confusion. Which person is very God and very man, yet one Christ, the only mediator between God and man.
→ VIII.2 · Of Christ the MediatorVIII.3 The Lord Jesus, in his human nature thus united to the divine, was sanctified and anointed with the …
The Lord Jesus, in his human nature thus united to the divine, was sanctified and anointed with the Holy Spirit above measure; having in him all the treasures of wisdom and knowledge, in whom it pleased the Father that all fulness should dwell; to the end that, being holy, harmless, undefiled, and full of grace and truth, he might be thoroughly furnished to execute the office of a mediator and surety. Which office he took not unto himself, but was thereunto called by his Father, who put all power and judgment into his hand, and gave him commandment to execute the same.
→ VIII.3 · Of Christ the MediatorVIII.5 The Lord Jesus, by his perfect obedience and sacrifice of himself, which he through the eternal Spirit once …
The Lord Jesus, by his perfect obedience and sacrifice of himself, which he through the eternal Spirit once offered up unto God, hath fully satisfied the justice of his Father, and purchased not only reconciliation, but an everlasting inheritance in the kingdom of heaven, for all those whom the Father hath given unto him.
→ VIII.5 · Of Christ the MediatorVIII.6 Although the work of redemption was not actually wrought by Christ till after his incarnation, yet the virtue, …
Although the work of redemption was not actually wrought by Christ till after his incarnation, yet the virtue, efficacy, and benefits thereof were communicated unto the elect, in all ages successively from the beginning of the world, in and by those promises, types, and sacrifices, wherein he was revealed, and signified to be the seed of the woman which should bruise the serpent's head, and the Lamb slain from the beginning of the world, being yesterday and today the same, and forever.
→ VIII.6 · Of Christ the MediatorVIII.7 Christ, in the work of mediation, acteth according to both natures; by each nature doing that which is …
Christ, in the work of mediation, acteth according to both natures; by each nature doing that which is proper to itself; yet, by reason of the unity of the person, that which is proper to one nature is sometimes, in Scripture, attributed to the person denominated by the other nature.
→ VIII.7 · Of Christ the MediatorVIII.8 To all those for whom Christ hath purchased redemption he doth certainly and effectually apply and communicate the …
To all those for whom Christ hath purchased redemption he doth certainly and effectually apply and communicate the same; making intercession for them, and revealing unto them, in and by the Word, the mysteries of salvation; effectually persuading them by his Spirit to believe and obey; and governing their hearts by his Word and Spirit; overcoming all their enemies by his almighty power and wisdom, in such manner and ways as are most consonant to his wonderful and unsearchable dispensation.
→ VIII.8 · Of Christ the MediatorIn the Westminster Shorter Catechism
Q.21 Who is the Redeemer of God's elect?
The only Redeemer of God's elect is the Lord Jesus Christ, who, being the eternal Son of God, became man and so was, and continueth to be, God and man in two distinct natures, and one person, forever.
→ Open Q.21Q.22 How did Christ, being the Son of God, become man?
Christ, the Son of God, became man, by taking to himself a true body, and a reasonable soul, being conceived by the power of the Holy Ghost, in the womb of the virgin Mary, and born of her yet without sin.
→ Open Q.22Q.25 How doth Christ execute the office of a priest?
Christ executeth the office of a priest, in his once offering up of himself a sacrifice to satisfy divine justice, and reconcile us to God; and in making continual intercession for us.
→ Open Q.25