So are you saying that a working definition of Christian would be the acceptance of the Gospel of Jesus Christ as the foundation of belief, with the rest of Scripture supporting rather than redefining it?
A scriptural definition—Yes. Our theology must be in concert with the Gospels. A theology based on the Gospels will correctly interpret the rest. The Gospels represent the God's full revelation to man. Without the Gospels, there is no Christianity. Te rest of the scriptures become meaningless.
There is a reason why only the Gospels are on the altar. There is a reason why everyone stands when only when the Gospels are read (at least in TLM), or why only clergy read form the Gospels.