Solus Christus or In Christo solo (Latin in + ablative, sōlō Christō, meaning “in Christ alone”) is one of the five solae that summarize the Protestant Reformers‘ basic belief that salvation is by faith in Christ alone.
Through the atoning work of Jesus Christ alone, apart from individual works, Christ is the only mediator between God and man. It holds that salvation cannot be obtained without Christ.
This is in opposition to Catholic and Orthodox doctrine which Mary, mother of Jesus is also a mediator between God and humanity (Mediatrix).
As the foundation of the “solus christus” doctrine, various biblical verses can be invoked according to theologians.
John 14:6 – “Jesus replied: I am the way, the truth and the life. No one comes to the Father except through me.”
First Epistle to Timothy 2:5 – “Because there is only one God, and only one mediator between God and men, the man Christ Jesus.”
Acts 4:10-12 – “May all of you and all the people of Israel know that this happened in the name of Jesus Christ of Nazareth […] And there is no salvation in anyone else; for there is no other name under heaven given among men by which we must be saved”
“This office of mediator between God and man is proper only to Christ, who is the prophet, priest, and king of the church of God; and may not be either in whole, or any part thereof, transferred from him to any other.” – London Baptist Confession
Christ alone (Solus Christus, Solo Christo), is one of the five Solas of the Reformation. It emphasizes that the death and resurrection of Jesus Christ is not only necessary for salvaiton but sufficient to save to the uttermost. That no amount of human works or merit can contribute to Christ’s finished priestly work. Christ’s all-sufficiency means, by implication, that we are insufficient of ourselves. Indeed the Scripture says “Not that we are adequate in ourselves to consider anything as coming from ourselves, but our adequacy is from God.” (2 Corinthians 3:5)
“I must listen to the gospel. It tells me not what I must do, but what Jesus Christ the Son of God has done for me.” – Martin Luther
“The heart of the gospel is not about us. The heart of the gospel is Christ for us (Christus pro nobis). This was the essence of Paul’s message: that Christ came for us, to do for us what we could not and would not do. He obeyed. He was crucified. He was raised. He is ascended. He is returning. The medieval church turned the gospel into a message about what Christ is doing in us, by grace, in sanctification, and about what we must do to do our part in order to benefit: cooperate with grace. The good news is that we have no part, not in this story. We’re recipient. We’re beggars; we’re not contributors to the story.” – R. Scott Clark
“Christ stepped in, took the punishment upon himself and bore the judgment due to sinners. With his own blood he expiated the sins which made them enemies of God and thereby satisfied him…we look to Christ alone for divine favour and fatherly love! …Hence Christ is called “King of peace” (Is. 9:6) and “our peace” (Eph 2:14) because he quiets all agitations of conscience. If we ask the means, we must come to the sacrifice by which God has been appeased. For anyone unconvinced that God is appeased by that one atonement in which Christ endured his wrath will never cease to tremble. In short, we must seek peace for ourselves solely in the anguish of Christ our Redeemer.”
– John Calvin, Institutes of the Christian Religion 2.16.2
Why is solo Christo important?
Solo Christo, or solus Christus, is one of the five solas (or solae) that have come to summarize the key issues of the Protestant Reformation. Solo Christo means “Christ alone” in Latin. The other four solas are sola scriptura (“Scripture alone”), sola fide (“faith alone”), sola gratia (“grace alone”), and sola Deo gloria (“for the glory of God alone”). Each one of these doctrines is vitally important. To dispatch with any one of them will lead to error and a false gospel that is powerless to save.
When the Reformers insisted on solo Christo, they affirmed that we are saved by Christ alone, apart from the merit of any other person. Jesus alone is the King of kings (Revelation 19:16). He alone is our High Priest (Hebrews 4:14). He alone is our Redeemer (Galatians 3:13) and the sole Mediator between God and man (1 Timothy 2:5). The attempt to usurp or share in those positions is a blasphemous arrogation. Assigning those roles to someone else (such as Mary) is equally improper. It is Christ and Christ alone who saves.
It is not our righteousness that saves us; it is Christ’s alone. “He saved us, not because of righteous things we had done, but because of his mercy” (Titus 3:5). “This righteousness is given through faith in Jesus Christ to all who believe” (Romans 3:22). Whatever good works we do and however faithful we are, in the final analysis “we are unworthy servants” (Luke 17:10). Christ and Christ alone is the Worthy One (Revelation 5:9). Solo Christo.
From beginning to end, the gospel uplifts Christ and Christ alone. He is the One who came from heaven to seek the lost (Luke 19:10). He is the One who obeyed the Law perfectly. He is the One who was crucified, and He is the One who rose again. We are the grateful recipients of His bounty. We are the beggars, and He is the Benefactor. We are the lepers, and He is the Healer. We are turmoil, and He is Peace. Solo Christo.
The gospel is not a message of what we must do for God; the gospel is the good news of what God has done for us. Salvation is not essentially about us; it is about Jesus. Solo Christo. In all things, Christ must have the supremacy (Colossians 1:18), and the Reformers restored that biblical doctrine to the church. As Luther wrote, “I must listen to the Gospel. It tells me, not what I must do, but what Jesus Christ, the Son of God, has done for me” (Martin Luther, Commentary on Galatians, Chapter 2, Verses 4–5).
This portion is borrowed from https://www.gotquestions.org/solo-Christo.html.