Going by that idea, I could make up a tradition that says Jesus was a one-legged homosexual vegetarian, practice it for a while, and then say it's true based solely on the fact that some people believe it- regardless of what the new testament says.
Furthermore, it was tradition for thousands of years that said the Earth was flat. Boy, were they wrong.
Actually, this has proved to be a legend made of whole cloth. A reputable scholar investigated this and found that virtually no one from the time of the ancient Greeks on beleived in a flat earth...if you'd like I can find the refs.
And as far as the traditions you can invent, you're exactly right. Traditions can be invented and are every day. That's why we go back to the earliest documents we can find and determine what was believed 2000, 1500, 100, 500 years ago. If we see an unchanging, rock-solid consistency from the beginning of Christianity right on up to the present, it's a clue we're not dealing with some fad of the moment. Our argument is that right from the start there's a consistency of Christian writing about Mary's perfect holiness...even among churches that haven't seen eye to eye for thousands of years (Catholic, Orthodox, Monophysite).
"Going by that idea, I could make up a tradition that says Jesus was a one-legged homosexual vegetarian, practice it for a while, and then say it's true based solely on the fact that some people believe it- regardless of what the new testament says."
The Anglicans already have, haven't they? Or maybe that was just bishops?
No, the Catholic understanding of Tradition as binding doctrine (as opposed to traditions) is that it must always have been believed by Christians - "always and everywhere" or ordinary and universal.
This is the same rationale that the Church applied to the Scriptures to determine which books were canonical.
"Furthermore, it was tradition for thousands of years that said the Earth was flat. Boy, were they wrong."
This kind of tradition did not have Christ's promise about it, that He would send the Holy Spirit to lead His Apostles into all truth about it.
I don't know if you are a Christian or not, but wouldn't you consider you comment blasphemy? Even in jest?