However, to answer your question directly; yes your statement is incorrect. The principle is not based on faith. It is based on mathematics and tested experimentally.
That's what I thought you'd say.
You are wrong. Dead wrong. Anyone with a rudimentary understanding of theoretical physics, in general, and/or the theory of special relativity, in particular, can and will tell you that.
Don't take my word for it. Do some research.
I know from past experience that it is useless and futile to try to explain things to you, so I won't even try. Ask around, maybe someone can get through to you, but it wouldn't be an easy task.
Your "understanding" of the nature of scientific theories, empirical verification, etc., is so entrenched, yet so deficient, that you are probably beyond hope at this point.
You might try reading what Einstein himself has written with respect to this very question. He makes it very clear--to anyone capable of understanding his words, anyway.