“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 (Is. 42:1; Jn. 3:16; I Tim. 2:5; I Pet. 1:19-20).” Even as the Father has elected a certain number out of the human race to receive salvation, it was necessary also that he, with the willingness of the son, and the witness of the Spirit, to set apart the Son to be a mediator between the triune God and elect sinners (Cf. WCF. 3:3-6). Furthermore, it is within the context of the covenant of grace that this mediator would come and act on behalf of the elect (WCF. 7:3-4). As this covenant is seen in the history of progressive revelation, variously administered, even so the work of the promised mediator is also seen to progress to an ever increasing revelation. Jesus was born with a purpose, and to this purpose he was ordained. See also the WLC 32, 36, 38-42.
Such a mediator had to be able to represent both parties, so that this is but one reason for the incarnation of the Son of God. When he was born under law, of the virgin Mary, and when he was of age, in his case 30, he was tested in the wilderness for 40 days of the devil, after his anointing of the Spirit. In this trial, the devil tested the Messiah on all three offices for which he was anointed, in his one person. He was first of all tested with respect to his office as a prophet, whether the word would be his first and only axiom of all thought and practice (Mt. 4:1-4). Next the devil tested him on how he would serve as the temple Priest of God’s presence (Mt. 4:5-7). Finally, the devil tempted him with the promise of all the kingdoms of the world, but he chose the path to his Messianic reign over all the kingdoms of the world in the worship and service of the LORD of the covenant.
The same progressive revelation coming to fruition in the Son, is given proof in the covenantal prologue of Hebrews 1:1-4, he who would follow in covenantal succession to the last of the old covenant administrations of grace in David. The son was in fact he by whom the prophets spoke, and who created the ages in which they spoke in this progressive revelation of himself (vv.1-2). The last days of the old covenant came to completion when the son ascended to his rightful place at the right hand of the Father, when, as our Great High Priest, “He had by Himself purged our sins.” He then, having fulfilled these two prerequisites, sat to reign as the Prophet-Priest-King (v. 3). The fact is, in the old testament, it was taught that only the Messiah could occupy all three offices in his one person as God and man (). His was the Son’s inheritance, all contained in his name – Jesus the Christ.
“The Prophet (Acts 3:22), Priest (Heb. 5:5-6), and King (Ps. 2:6; Lk. 1:33); the Head and Saviour of his Church (Eph. 5:23); Heir of all things (Heb. 1:2); and Judge of the world (Acts 17:31).” These truths find even fuller expression in the Shorter (23-26), and Larger Catechisms (42-45). As regards the role of “Judge of the world (Acts 17:31),” it is important to note that Jesus as the righteous One, was also ordained to this task, it being committed to him by the Father (Jn. 5:22). “Head and Saviour” of course refers to his role as mediator being within the context of the covenant of grace (Ch. VII.), and to whom the Father “did from all eternity give a people to be his seed (Ps. 22:30; Is. 53:10; Jn. 17:6), and to be by him redeemed, called, justified, sanctified and glorified (Is. 55:4-5; I Cor. 1:30; I Tim. 2:6).” Section V refers back to the rest of this chapter as dealing with reconciliation (Cf. Hodge, 133-137).