Carlos, a Peruvian student, asked this provocative question during my course on grace. My answer:
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Tough question! Tough because it can be approached from two opposing angles.
Catholics and protestants alike hold firmly to three important creeds from the first centuries of the Christian era. They define foundational Christian teaching particularly about what God is like. When it comes to doctrine of God, Rome is rock solid.
These creeds are: The Apostle’s Creed (circa early second century; the Nicene Creed (325 A.D.) defining the deity Christ; the Athanasian Creed (fifth century) defining the Trinity. These are called the Ecumenical Creeds because they are held in common between Catholics and Protestants. [1]
The point: is it reasonable to deny the term “Christian” to an entity that affirms these creeds and worships the Christian God?
Those who hold this view agree that worship of the Christian God is a necessary requisite but argue it is not sufficient. The church must also proclaim the biblical gospel of justification by faith alone in Christ alone.
This position corresponds to Galatians 1:6-9, in which Paul sharply declares that anyone who preaches any other gospel than what he preached is anathema (cursed of God.) Throughout the rest of the book, Paul describes his gospel as justification by faith alone in Christ alone, apart from works.
Rome emphatically denies this definition and pronounces anathema on anyone who teaches it.
The point: If a Christian is defined as a person saved by grace through faith alone in Christ alone, is it reasonable to consider someone a Christian who denies that?
Carlos, I know you were hoping for a definitive answer but we must leave it as a matter of conscience.
Blessings,
Roger
[1] The term Ecumenical in this context has nothing to do with the modern Catholic-Protestant ecumenical movement.