Christianity is defined by the set of beliefs espoused by Christ Himself. To my mind, this is not as dependent on lineage or continuity as it is on faithful interpretation of the Word of God. In short, you’re unlikely to sway my opinion by stating that there was a different Biblical interpretation in 1400AD. In my opinion, the men of 1400AD were just as likely as we are to be wrong in their interpretation. And, tradition can often justify practices that are not otherwise justifiable.
The church established by Christ is the Christian church — in many, if not most, respects, we (Catholic and Protestant) are one church of believers in the divinity of Christ. The lineage of every current Christian denomination can therefore be traced back to Christ Himself.
To the extent that there are legitimate disagreements among Christians as to doctrine, I believe every believer is capable of educated interpretation of the scripture.
Some are certainly more educated than others, and, in that respect, the Vatican is a collection of some of the more educated Christian theologians in the world. Some opinions are certainly worthy of greater respect than others.
As a Baptist, I often listen to the Vatican on theological matters. They are not infallible by any stretch — but they are educated enough to be worthy of consideration (even when I disagree).
SnakeDoc
A document of any type is only as valuable as its comprehensibility, its ability to be understood and interpreted. We both believe the Bible to be the inspired, infallible Word of God. I would submit that a document that purports to be infallible needs an infallible interpreter. Otherwise, its very infallibility is rendered useless. Jesus does not generally come down personally, in visible, human form, to resolve Scriptural disputes. He has delegated that responsibility to His Church, and guides it to correct interpretation through His sending of the Holy Spirit. There is still an interpretation that is entirely authentic, but is delivered through the human agency He Himself has delegated.
This is only fitting, really, since we are to walk by faith and not by sight. It takes faith (and trust in that faith) to follow and embrace all that the Church teaches in its Deposit of Faith as coming from God Himself. But there is really no viable alternative short of God personally taking us individually by the hand and walking us through everything. But, for God to reveal Himself that explicitly would quite radically remove from us the free will to accept or reject Him, wouldn't it? Who could be free to choose Him or not, if He physically taught each one of us in this life? We would have no freedom at all. Which is why He teaches through the agency of His Church, established, as I already said, to teach all nations to the end of time all things that He and the Holy Spirit have taught, either directly from the Mouth of Christ Himself, or through the inspired oral and written teachings of the Apostles. In other words, to teach the Deposit of Faith, which can be found intact and entire from Pentecost until the end of time.