“objective proof that it is”
Objective proof is inductive reasoning (a posteriori) and that only renders “probable” conclusions, therefore, the correct statement is “I believe....”. “Pure Christianity” is a belief system and since this is a fallen world and we are fallen people saved by grace, we can’t know that our particular faith system is “pure”. Paul says, “we see through a glass darkly...”
Belief is something you have that you have not seen and hope it is whatever you expect it to be. I don't have to precede my statement with "I believe" because it is a known fact, relative or absolute, it makes no difference. The fact is that we are the least changed, but rather represent a continuum of doctrine worship as the Church that canonized the NT.
Now, if you wish to accuse the Church of the 4th century of doctrinal impurity, you certainly are entitled to your opinion, but that doesn't change the fact that such an accusation would be hard to reconcile with that Church's ability to infallibly select and produce the Christian canon.
Staying that a posteri arguments are only "probable" is certainly true in some cases. In others it is not. Do you doubt that Henry VIII lived? Do you think it's probably that he didn't?
Bottom line: my tagline is a lot more factual then claiming indwelling spirit.