Each of the Solas of the Reformation depends on the others, and as we think about Solus Christus, the sufficient, finished work of Christ on the cross, we cannot do so without first acknowledging that God's plan of dispensing grace (alone) through faith (alone) leads us to the person of Christ (alone). God's plan to save the world through the work of a Redeemer is not Plan B. There is a hint of this plan as far back as Genesis 3:15, just after the fall of man.
I will put enmity between you and the woman,
and between your offspring and her offspring;
he shall bruise your head,
and you shall bruise his heel.
A Messiah would be prophesied, and would come in fulfillment of those prophecies for those waiting for the true "consolation of Israel" (Luke 2:25). Christ's purpose in coming was to fulfill his role as Savior, Redeemer, and Lord. Jesus came to earth to die. Each of the gospels takes this as its central theme - his ministry leading us to who he is and his sufficiency as our substitute when he goes to die. The gospel of Mark is divided into two halves. The first half, three years of ministry. Second half, one week of his passion to the cross.
There is no other way to be saved than through Jesus Christ (Acts 4:12). Sin has separated us from God, and the only way to be reconciled is to be redeemed, to be "bought back." Only one man could die in the place of sinners that would give them eternal life. He had to be both man (to die in place of man, to really spill his blood) and God (to live a life free from sin, whose righteousness is now given to his followers in God's eyes). He was the perfect, spotless lamb, and offered himself as the once-for-all sacrifice (Hebrews 10:10) to appease our God of justice (who does not take sin lightly) and demonstrate the amazing grace of our God.
As John Stott reminds us, when you look up at the cross, you should say, "That should have been me." But you look again and say, "That will never be me." We owe nothing for our sin; Jesus lived the life we should have lived and died the death we should have died in our place. He is the ultimate substitute, and all we need to be redeemed.
In our modern world (and frankly, in the ancient world, as well) people search for ways to earn their way to God. We think things like "I'm a pretty good person, therefore, God will accept me." But think of the sin of Adam and Eve, who ate a piece of fruit they weren't supposed to. Their sin in rejecting God's care for them got them kicked out of the garden (a place of perfect peace) and caused death to enter the world. God deals with sin, and he does not consider our sin to be just an interesting foible that makes up our interesting character. It is against our very nature as God's good creation to sin at all. We've made a mess of ourselves, and there is no way out except through Christ. As we reflect on this doctrine, let us not forget that the claims of the Christian faith are matters of life and death. Let us, no matter our tradition, center ourselves on Jesus, the author and perfecter of our faith.