The rule of thumb for distinguishing between the scientific and the unscientific is what is called "falsifiability", the ability for a statement, a theory, or an hypothesis to be able to be proven false. If there is no way to prove something false, it may or may not be true, but it is not scientific. So says Sir Karl Popper, a philosopher of science. "So," they tell us, "since religion cannot be proven false, it cannot be scientific and should be excluded from the public square." You see, religions are generally held as a matter of faith, not evidence or reason, and thus cannot be falsified.
Recently someone was complaining about another's lack of "acting like a Christian". The other was pointing out the errors of another religion, and that, according to the first person, was wrong. "There is no 'one true religion'," she claimed. There you have it. Proof! Or, rather, falsification.
Christianity is unique in this regard. Most religions claim to be exclusively true. Agree with them and you're right; disagree and you're wrong. Simple as that. Logically this would require that at best only one of these religions was true, since they can't all be exclusively true. But, of course, it's not as black and white as that. It could be that the adherents of the religion in question are mistaken. It could be that what they understood as a claim to exclusivity was not such a claim. In this case, the religion with the exclusive claim could be true with the exception of that claim. Someone goofed.
What we have here, then, is a falsifiable point. If it can be proven that religions with the claim to exclusivity are not exclusive, you can eliminate that those religions. Or, at least, most of them. But, of course, there is that "They were mistaken" caveat.
Christianity doesn't get that "out". Christianity is under the gun here. Christianity, if you recall, is based on Christ. Beyond that, it is based on Christ as God. Very important. If that isn't true, Christianity isn't true. So we have Jesus's own words on the topic of exclusivity. "I am the way, and the truth, and the life. No one comes to the Father except through Me." (John 14:6) So this is simple. If it is true that there is "no 'one true religion'", Christianity has been falsified. Maybe not Judaism. Maybe not Islam. But certainly Christianity. Why? Because 1) it was Christ who said it and 2) Christ is God. If He was wrong, we're done. That's "falsifiability".
Lots of people balk at the exclusive claim of Christianity. It's mean-spirited and closed-minded. It's not inclusive. "We need more inclusivity and we will exclude you if you're not more inclusive." It's not like we have an option here. We can hold to Christ's words that He is the only way and be exclusive, or we can be inclusive and reject Christianity outright. There are no other choices. So, easy challenge. Prove that there is no "one true religion" and you've falsified Christianity.
I thought the complaint was ironic. "You're not acting like a Christian" followed by "There is no 'one true religion'." That is, logically, "You're not properly following your false religion." I wonder how that works?