If we believe that Scripture is our only infallible and ultimate authority for faith and things of faith, it follows that all other theology must flow from Scripture. When when theology comes from extra-Biblical writing and/or historical writings and/or tradition and cannot be backed up by Scripture, the theology must be discarded.
Solus Christus - Christ alone. By Christ's finished work on the cross, alone, are we saved.
There is no other mediator (or mediatrix) (1 Timothy 2:5 For there is one God and one mediator between God and men, the man Christ Jesus...
There is no other Redeemer (or redemptress) (Hebrews 9:15 For this reason Christ is the mediator of a new covenant, that those who are called may receive the promised eternal inheritance—now that he has died as a ransom to set them free from the sins committed under the first covenant. )
John Calvin said in the Institutes of the Christian Religion, "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!"
The Heidelberg Catechism, Question 30 asks, "Do such then believe in Jesus the only Saviour who seek their salvation and happiness in saints, in themselves, or anywhere else? They do not; for though they boast of him in words yet in deeds they deny Jesus the only deliverer and Saviour: for one of these two things must be true that either Jesus is not a complete Saviour or that they who by a true faith receive this Saviour must find all things in him necessary to their salvation."