How we understand the person of Jesus determines our understanding of his work, and vice versa. Imagine, for the purposes of discussion, that Jesus Christ is merely a man. Certainly a very gifted man, for God was — either by a special act of conception or by adoption  — his Father. Such a man would not be God, and any divine titles or prerogatives would be his only by being conferred by God. Yet, as Christadelphian writer Harry Tennant puts it, “How great is his work! From the lowly beginnings when he shared our nature, he overcame the temptations of sin, entered and conquered death, and has ascended in immortality and abounding glory to the right hand of God in heaven.” 
Let’s think this through for a moment. Jesus shared our nature, with all its frailties, temptations and proneness to sin — yet he never sinned. Not once. Not as a toddler, not as a child, not as an adolescent, not as a man. He remained holy, harmless, undefiled in all his thirty years, in which it must be acknowledged that he grew and developed and learned obedience (Luke 2:52; Heb 5:8). As discussed in The Paradox of Certainty this is simply not possible for a mere human. He had to have divine aid to achieve this. Yet as soon as we allow enough divine influence, enabling or coercion to achieve sinlessness, do we retain true humanity? The Bible’s answer to this conundrum is that Jesus was both divine and human, God and man, the Word made flesh.
Second problem: In what way could a mere human bear the sins of the world, carry them to the cross, bear the wrath of God on our behalf and totally destroy sin? For it is clear from Isaiah 53 and elsewhere that he took upon himself our very sins and the punishment for them. But if Christ was merely human, substitutionary atonement becomes problematic, for it makes one who was no different from the rest of us take the blame for the sins of others in what would be a gross miscarriage of justice. Effectively God saying, “You humans deserve death, every one of you, because of your sins. But in my love and generosity, I’ll lay it all on this one man here, and he’ll take the blame, while you all get forgiveness.” The logical counter view is that Christ’s sacrifice was representative only, not substitutionary. As Christadelphian Robert Roberts wrote, “This passing by of our sins is the act of His forbearance; that no debt of ours has been paid or can be paid; that what the death of Christ has done has been to declare God’s righteousness that we may, by taking part in it, receive God’s forgiveness through him… The idea that Christ has borne our punishment and paid our debts, and that his righteousness is placed to our credit, and that the only thing we have to do is to believe it, is demoralising… He only is righteous who doeth righteousness… we have to ‘work out our own salvation’ by a ‘patient continuance in well doing.’”
In this view, Christ’s death is held up as the example of what humans deserve for their sin and if sinners acknowledge this and undertake to live righteously, God will forgive them for Christ’s sake. “The cleansing result of the atonement,” continues Roberts, “is dependent upon our compliance” (i.e. our works). Christ’s death was a “declaration of God’s righteousness” and in acknowledging that is what sin deserves, we effectively “apologise” to God. Salvation then rests on one’s own effort, which robs the Christian of any assurance, for who can be sure of persevering to the end? Christ’s overcoming of sin is put down to his own efforts, albeit with assistance from his Father (but not enough to make Jesus in any way divine). The extent and nature of that assistance has been the subject of some debate, but put simply, “The Bible shows us he had to fight to overcome (sin). It was a struggle, but Jesus was victorious.” 
So in this view, all the richness of the atonement is lost; where is the justification, reconciliation, propitiation, ransom? And where is the assurance that the sinner is justified and faces no condemnation (Eph 2:8; Rom 8:1, 30; 2 Cor 1:22; 1 John 5:13)? Where is the certainty, if Christ is merely human and there is every possibility he could have failed, but for the supernatural intervention that kept him from sinning?
“How we understand the relationship between Father and Son influences how we understand God’s plan of salvation,”  admits Christadelphian Richard Benson, but beyond this we part company, for as a consequence of their devaluing the person of Christ, non-trinitarians devalue his work. By denying Christ’s divinity, Christadelphians cannot accept a substitutionary atonement. Because his work is seen as only representative, not substitutionary, it is incomplete and conditional; it requires the believer to conform to what Christ represents. It is adding to Christ’s work, declaring it to be insufficient without human effort.
In contrast, understanding both the divinity and humanity of Christ allows acknowledgement that the atonement is all God’s initiative and wholly the work of God. Christ’s work is complete, it is finished, enabling Scripture to give us the assurance that those in Christ are already justified, not of works, lest anyone should boast. When we understand Christ to be God with us, God incarnate, that is, manifested in the flesh (Matt 1:23; John 1:14; 1 Tim 3:16), the gospel of our salvation in Christ takes shape beautifully.
The Apostle Paul expresses it this way in 2 Corinthians 5:19, “In Christ God was reconciling the world to himself, not counting their trespasses against them, and entrusting to us the message of reconciliation.” The reconciliation between God and mankind took place at God’s own initiative, in the person of his Son. Just as Father, Son and Spirit worked together to effect creation, so the work of salvation is wholly the work of God; Father, Son and Spirit. In this, the person and work of Christ is pivotal.
Paul, writing to the Ephesians, presents Christ as the conduit for “every spiritual blessing;” God chose us in him and predestined us to adoption though him. Through him we have grace and redemption; in him we were chosen. The mystery of God’s will, which he purposed in Christ, is to bring all things in heaven and on earth together under one head, even Christ” (Eph 1:3–12).
The Son was sent by the Father, and he went freely; God himself provided the Lamb, God himself entered creation to redeem it. The consequences of undervaluing Christ, then, are grievous. If Christ is not God, he is a mere representative of humanity and salvation can only rely on our ability to identify with and emulate him. But if Christ is God, then salvation is fully effective because it does not rely on the strength of humanity. The true humanity of Christ is not in question here; the Trinitarian concept of atonement and salvation fully embraces and requires the true humanity of of Christ, but it also requires him to be more than a regular human being. The doctrine of the incarnation, the Son taking on humanity and having two natures perfectly combined, takes care of all the difficulties raised by the alternative model described above. Being fully human, Christ could be tempted. He shares our nature and hence is qualified to judge, to bear our sins and to act as mediator (Heb 2:14–18; 4:15). But by being God he can himself step in as the perfect substitutionary sacrifice and do what we could not do for ourselves; live a sinless life, defeat sin in the flesh and bear the sins of the world. As John Stott explains,
“The saving initiative was taken by God in his love. It is he who has propitiated his own wrath, redeemed us from our miserable bondage, declared us righteous in his sight and reconciled us to himself… The death of Christ was the atoning sacrifice because of which God averted his wrath from us, the ransom price by which we have been redeemed, the condemnation of the innocent that the guilty might be justified, and the sinless One being made sin for us (Rom 3:25; 1 Pet 1:18–19; Rom 8:3, 33; 2 Cor 5:21).”
If a human sacrifice per se was all that was required for representational atonement, effectively any human being empowered by God so as to not sin would have sufficed as the atoning sacrifice, the propitiation, the substitute sin-bearer. Roberts again: “Because being born of Adam’s condemned race, and partaking of their condemned nature, Christ was made subject, equally with them, to the consequences of Adam’s transgression. Therefore his public execution was a public exhibition of what was due to a man from God. It pleased God to require this before inviting men to reconciliation through the man in whom this vindication should take place. 
But is this really good enough? Does it do justice to the richness yet simple clarity of Bible teaching? If Christ was not God, substituting for us, if he was merely a representative man (albeit imbued with divine traits) salvation would come down to works, to human effort, something to be emulated and achieved. It would take the initiative from God and be a human accomplishment. Ironically, the less divinity we attribute to Christ, the less the atonement becomes a work of God, to the glory of God alone.
But the atonement IS a work of God, from beginning to end, which is why it is efficacious; there was no mere man able to do it, so his own arm brought salvation (Isa 59:16) and God himself provided the Lamb. God reconciled us to himself in Christ (2 Cor 5:18–19) and purchased us with his own blood (Acts 20:28). Christ gives eternal life (John 10:28) and with the Father and Holy Spirit, sanctifies (1 Cor 1:2, 1 Thess 5:23, 2 Thess 2:13).
Scripture is clear, repeatedly, that Jesus Christ bore our sins. He didn’t just represent them, he bore them, carried them, nailed them to his cross and died for them (Col 2:14; Isa 53; 1 Pet 2:24). The antitype of the scapegoat, the Lamb of God, Jesus took away the sins of the world. How could a single human being bear in himself the sins of the world and the punishment for them? How could any finite creature? Salvation is of God, through and through, from the initiative within God’s eternal counsels, through the incarnation, to the redemptive event itself and, ultimately at the consummation of all things. God himself was the Just One, requiring propitiation and also the Justifier (Rom 3:25–26). The Savior had to be Christ, the LORD (Luke 2:11).
Indeed, how great is the work of Christ, because how great is his Person. In the words of Casting Crowns, “Not because of who I am, but because of what you’ve done. Not because of what I’ve done but because of who you are…” 
1 Unitarians believe that Christ was a regular man, the natural son of Joseph as well as Mary. Many would also hold to an Adoptionist position, that God anointed this man Jesus with the Holy Spirit and conferred upon him Sonship. Christadelphians believe that Jesus was purely human, however his conception was a work of the Holy Spirit in Mary, such that he was also the Son of God, but not intrinsically divine.
2 Harry Tennant, The Christadelphians: What they believe and teach (Birmingham: The Christadelphian, 1998), 100–101
3 Robert Roberts’ argument is set out in detail in his short work The Blood of Christ, which he wrote in 1895, and has been continuously republished, (Birmingham:The Christadelphian, 2006) and still held to be the definitive statement on Christadelphian beliefs concerning the atonement.
4 John S Roberts, The Bible, Lord Jesus and You, 54. (The Christadelphians, no publication date given, circa 1990s.)
5 Richard Benson, “Monotheism and the Atonement,” in Gaston, ed. One God, The Father (UK: Willow, 2013), 257.
6 John Stott, The Cross of Christ, 2nd ed. (Nottingham: IVP, 2006), 235–6.
7 Robert Roberts, The Christadelphian Instructor (1891) Article 55. Reprinted West Beach SA: Logos Publications, 1985 and still in use as a catechism throughout the movement.
8 Who Am I? Casting Crowns 2004