So, like virtually every word, its meaning varies with context.
"I am
the Scott McClare." Exclusivity. So far as I'm aware, my name is unique. (Merriam-Webster definition 1b) Similarly, "Jesus the Christ." There's only one. (
Merriam-Webster definition 1b)
"Please pass the salt." Specificity. It's implicitly clear that I want
my salt, in the shaker on my table, not someone else's salt, the pepper, or the butter. (definition 1a)
Vlad the Impaler, Jack the Ripper, Edward the Confessor, etc. Again, specificity. It identifies the individual you mean by describing a particular characteristic he is known/notorious for. (definition 2b[2])
John the Baptist falls into the last category. He is characterized by his practice of baptizing people. When you say he has an exclusive licence to the term "Baptist," you are equivocating on the definition of the word "the." it has a wide semantic range for such a simple word--the entry (i.e. the only entry for the word
the--exclusivity) in my desktop dictionary takes up a quarter of the page (by which I mean its specific page, 1444, not that the dictionary has only one page--specificity. See what I mean?)
And in any case, the title "Baptist" used by John doesn't mean the same thing as the title "Baptist" used by a group of English Protestants. So you're ranting about nothing anyway.