After working through Sola Gratia (By Grace Alone) and Sola Fide (By Faith Alone), we come to Solus Christus - Christ Alone.
The Solas of the Reformation are related to one another and effectively form a unit. We cannot speak of Saving Faith without speaking of Grace and of course the source and object of salvation is Christ. He is the very foundation and substance of the gospel. We speak of His Person - both the Son of God and Son of Man united in One Person. But we also speak of His Work - his role as the Second Adam, his death on the cross, and his resurrection signifying his triumph over death. Through Christ we are reconciled unto God, forgiven of our sins, and restored to the fellowship lost at the time of the Fall in Eden.
Again, the Solas exist in the context of the Reformation's controversy with Roman Catholicism. Rome had over the centuries erected a 'sacerdotal' system of priesthood, a clerical aristocracy that in practical terms controlled the means or administration of salvation. The doctrine of 'Apostolic Succession' meant their validity and authority was found in a chain of ordinations going back to the apostle Peter.
Catholicism developed a system of mediators - beyond the priesthood, they elevated Mary and the saints. Remember in the Roman system, a saint is one who is in heaven and thus (they argue) has access to the throne of God. The majority of Christians remain in Purgatory. Praying through the saints (they argue) is like asking a fellow Christian to pray for you, but in practice such prayers became indistinguishable from worship, and the cult of saints was turned into a kind of pantheon - a panoply of lesser gods. Mary's veneration reached such heights that for years she has functioned as a Co-Redemptrix alongside Christ - an issue very much alive in contemporary Catholic debate.
Contrary to Rome, the Reformers argued for the priesthood of all believers (Exodus 19:5, 1 Peter 2:9), and that access to God is through Christ alone. He is the only mediator of the New Covenant (1 Timothy 2:5, Hebrews 9:15, 12:24) and there is salvation by none other (John 14:6, Acts 4:12) - a message that flies in the face of both Rome's claims and the world's intuitions.
Rome's effective worship of the saints and of Mary are without Scriptural warrant and were unknown even to the Early Church. These errors developed over the centuries and the Reformers sought to return to the Bible and re-ground the gospel in the Person and Work of Christ Alone. We read in 2 Corinthians 1:20 that all the promises of God (from the Old Testament) are affirmed and confirmed (yea and amen) in Christ. Every symbol, type, picture, and gospel-related episode point to Christ and find their fulfillment in Him. He is the true Prophet, Priest, and King - a threefold office magnificently revealed in the epistle to the Hebrews.
On the road to Emmaus (Luke 24:25-27) Jesus lays out the whole of the Biblical structure - what is sometimes referred to as Redemptive-History, the plan of salvation revealed by God from the Fall to the coming of Messiah, and the Age to Come. As is made clear, Christ is the central focus of all Scripture, of all God's revelation.
Christ fulfills the entire Old Testament system and as such we no longer have a physical temple or priesthood - or even a holy land to call our own. The Kingdom Christ preached is heavenly and finds its fulfillment in the age to come - after His glorious return. By means of the Spirit we even now have access to that Kingdom and participate in its life.
The Church plays an important and critical role in all this but Rome's theology is a distortion that undermines and obfuscates the gospel message and its centrality in Christ.