Who do you have in mind? Don't you think your argument would have to depend on what is Biblical Christianity, or, better yet, what constitutes the Bible. Given that the mainline Protestant canon deviates from the Church canon, obviously your argument cannot be applied to the Church. Our Bibles are not the same.
Other sects/cults use additional "scripture" to justify their beliefs "biblically." Thus, Gnostics used additional "gospels" and LDS use the Book of Mormon in addition to the OT and NT. They will tell you that their Christianity is scripturally "sound." On what basis, other than your personal preference of what constitutes scriptures, can you say that they should "no longer be called 'Christian'?
No Protestant can with any certainty say that another Christian is not a "true" Christian. That is the problem with relativism which the Reformation brought in. There is no truth; truth is unknown and if truth is unknown, so is God. In Protestantism, no denomination believes it has true and fully revealed faith. Therefore no protestant "church" can hold to be the true Church of Christ.
The best you can do is find "core agreement." That is hardly enough. For at the bottom of the "core" belief all monotheists (Jews, Christians and Muslims) are "united" in one God! Under "core" belief, Mormons are Christians because they call Christ their Savior. Under "core" belief, mainline Protestants and Catholic/Orthodox Churches are one and the same, because we all agree that God is a simple Trinitarian Monad, and Christ is perfect God-Man (although it takes only to scratch the surface of these concepts to discover that Protestant Trinity and Christology are not the same as those of the Apostolic Church). Outside of those (on the surface) "core" beliefs we don't share anything else with Protestants/Baptists!
And, finally, the Protestants among themselves don't share more than "core" beliefs, all based on individual preference and personal interpretation of the a certain canon of personally chosen scriptures.
Merely because the RC considers itself under one Pope, does not mean that there is perfect unity within it. Similarly, the body of non-RC churches should not be seen as having disunity merely because they are in many denominations
It's not comparable. All Roman Catholics believe one and the same thing. There are no "core" Catholic beliefs they all share and then have a plethora of "other beliefs" they don't. Catholics in order to be Catholics by definition believe one and the same thing on all aspects of the Catholic faith. The same is true of the Eastern Orthodox. You can't be a Catholic/Orthodox "a little bit." It's an either or issue.
So, while our praxis may be influenced by our weaknesses and personal preferences, liberal or conservative leanings, our faith does not vary from person to person. Liberal Catholics still believe exactly the same thing the conservative Catholics believe. So, there is perfect unity in faith even if there is not always unity in praxis.
John Kerry cannot claim to be a Catholic and support infanticide (late term abortions), or abortions in general, and expect to be a Catholic. He is free to believe what he wills, but if those are his beliefs than he has excommunicated himself form the Church because there is no room for such abominations in the catholic and orthodox faith.
it is incorrect to speak of the "Protestant/Baptist world" as if it were a uniform entity
All heresy is the same, even if the devil is in the details. :) They are all united in their "core" denial of, and separation from the Apostolic Church.
The focus is not on the believer, but rather on God. Similarly, truth is not dependent upon mere individual interpretation, but rather upon the Spirit working within the individual, but tested against both scripture and other mature Christians
But never against the 2,000 year old cumulative knowledge and wisdom of the Church. You will go to "mature" Christians (mature by whose standards?), and rely on the nebulous working of the Spirit within (never mind that it is your word against mine), but will not trust the earliest Church Fathers who were the closest to the original Church and the Apostles!
Protestantism is a fortress religion. Everyone shuts himself in with the Spirit, real or imaginary, and distrusts everyone else.