Someone put it quite aptly: "If you can't show it, you don't know it".
The general idea is that, since it's in our interests to believe things which are true, then belief in claims which have not been demonstrated to be true is not rationally justified.
We operate on certain standards of evidence which have been demonstrably proven to be the most reliable method available to us currently when determining what is true or what to believe (to varying degrees of certainty, of course). Further, it has also been demonstrably proven that not applying or disregarding these standards leads to results which are incompatible with the truth.
Using faith instead of these standards, is one such example of a method that provides results which are demonstrably incompatible with the truth.
If our goal is to have as accurate an understanding of reality/truth as possible (by believing as many true things, and as few false things, as possible), then by definition we must apply the same proven standards to all claims when deciding what to believe in order for our belief/knowledge to be rationally justified.
Disregarding those standards in order to believe something which does not meet them is nothing more than special pleading and intellectual dishonesty.
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