Why was it requisite that the Mediator should be God and man in one person ?
It was requisite that the Mediator, who was to reconcile God and man, should himself be both God and man, and this in one person, that the proper works of each nature might be accepted of God for us, and relied on by us,1 as the works of the whole person.2
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 Matt. 1:21, 23; Matt. 3:17; Heb. 9:14
Matt. 1:21 And she shall bring forth a son, and thou shalt call his name JESUS: for he shall save his people from their sins.
Matt. 1:23 Behold, a virgin shall be with child, and shall bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us.
And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased.
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 1 Pet. 2:6
Wherefore also it is contained in the scripture, Behold, I lay in Sion a chief corner stone, elect, precious: and he that believeth on him shall not be confounded.
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