Why was our Mediator called Christ?
Our Mediator was called Christ, because he was anointed with the Holy Ghost above measure;1 and so set apart, and fully furnished with all authority and ability,2 to execute the offices of prophet,3 priest,4 and king of his church,5 in the estate both of his humiliation and exaltation.
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 John 3:34; Ps. 45:7
For he whom God hath sent speaketh the words of God: for God giveth not the Spirit by measure unto him.
Thou lovest righteousness, and hatest wickedness: therefore God, thy God, hath anointed thee with the oil of gladness above thy fellows.
2 John 6:27; Matt. 28:18–20
Labour not for the meat which perisheth, but for that meat which endureth unto everlasting life, which the Son of man shall give unto you: for him hath God the Father sealed.
Matt. 28:18 And Jesus came and spake unto them, saying, All power is given unto me in heaven and in earth.
Matt. 28:19 Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:
Matt. 28:20 Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, even unto the end of the world. Amen.
3 Acts 3:21–22; Luke 4:18, 21
Acts 3:21 Whom the heaven must receive until the times of restitution of all things, which God hath spoken by the mouth of all his holy prophets since the world began.
Acts 3:22 For Moses truly said unto the fathers, A prophet shall the Lord your God raise up unto you of your brethren, like unto me; him shall ye hear in all things whatsoever he shall say unto you.
Luke 4:18 The Spirit of the Lord is upon me, because he hath anointed me to preach the gospel to the poor; he hath sent me to heal the brokenhearted, to preach deliverance to the captives, and recovering of sight to the blind, to set at liberty them that are bruised,
Luke 4:21 And he began to say unto them, This day is this scripture fulfilled in your ears.
4 Heb. 4:14–15; Heb. 5:5–7
Heb. 4:14 Seeing then that we have a great high priest, that is passed into the heavens, Jesus the Son of God, let us hold fast our profession.
Heb. 4:15 For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin.
Heb. 5:5 So also Christ glorified not himself to be made an high priest; but he that said unto him, Thou art my Son, to day have I begotten thee.
Heb. 5:6 As he saith also in another place, Thou art a priest for ever after the order of Melchisedec.
Heb. 5:7 Who in the days of his flesh, when he had offered up prayers and supplications with strong crying and tears unto him that was able to save him from death, and was heard in that he feared;
5 Ps. 2:6; Matt. 21:5; Isa. 9:6–7; Phil. 2:8–11
Yet have I set my king upon my holy hill of Zion.
Tell ye the daughter of Sion, Behold, thy King cometh unto thee, meek, and sitting upon an ass, and a colt the foal of an ass.
Isa. 9:6 For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace.
Isa. 9:7 Of the increase of his government and peace there shall be no end, upon the throne of David, and upon his kingdom, to order it, and to establish it with judgment and with justice from henceforth even for ever. The zeal of the LORD of hosts will perform this.
Phil. 2:8 And being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross.
Phil. 2:9 Wherefore God also hath highly exalted him, and given him a name which is above every name:
Phil. 2:10 That at the name of Jesus every knee should bow, of things in heaven, and things in earth, and things under the earth;
Phil. 2:11 And that every tongue should confess that Jesus Christ is Lord, to the glory of God the Father.
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.1 It pleased God, in his eternal purpose, to choose and ordain the Lord Jesus, his only-begotten Son, to …
It pleased God, in his eternal purpose, to choose and ordain the Lord Jesus, his only-begotten Son, to be the Mediator between God and man, the Prophet, Priest, and King; the Head and Saviour of his Church, the Heir of all things, and Judge of the world; unto whom he did, from all eternity, give a people to be his seed, and to be by him in time redeemed, called, justified, sanctified, and glorified.
→ VIII.1 · 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 MediatorIn the Westminster Shorter Catechism
Q.23 What offices doth Christ execute as our Redeemer?
Christ, as our Redeemer, executeth the offices of a prophet, of a priest, and of a king, both in his estate of humiliation and exaltation.
→ Open Q.23Q.24 How doth Christ execute the office of a prophet?
Christ executeth the office of a prophet, in revealing to us, by his Word and Spirit the will of God for our salvation.
→ Open Q.24Q.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.25Q.26 How doth Christ execute the office of a king?
Christ executeth the office of a king, in subduing us to himself, in ruling and defending us, and in restraining and conquering all his and our enemies.
→ Open Q.26