Why is Christianity the only true religion?
Your post is one of those extra-topical questions I'm sure we'll see over and over to challenge …
"How can you guys believe this stuff is Truth?"
Simply stated, your question outside the purpose of this site. This is not what this site is about.
The scope and purpose of this site is to create a canon of knowledge ABOUT the subject of "Christianity". Obviously, we are not checking credentials at the door, but within the scope of this site, users generally do not expect to be challenged on their belief system and, frankly, asking them to justify their own existence is not the purpose of this site.
But let's dispel the notion that this has anything to do with the hot-button issues of "religion" or "freedom of speech" with a quick analogy — We have an Apple site for experts and enthusiast interested in Apple products. If somebody were to ask, "Why do y'all think Apple is the best when so many more Windows and other users think otherwise…?" — it would be closed as "not constructive". The whole issue is largely irrelevant in the context of that site.
It's the same thing here. That's how you build Q&A sites. That's how you form and preserve community.
So, to answer your question specifically…
Isn't almost everything open to debate and different arguments?
No. In some other discussion or debate forum; maybe, where those issues are up for discussion… but this is not an open discussion forum. This is a tightly-scoped Q&A site to ask questions regarding the canons and teachings of Christian faith. That also includes good-faith questions about their Christian lifestyle where folks are trying to rectify their day-to-day issues with the ideals of those teachings.
We talk about building these sites for "experts" and "enthusiasts." But those terms also apply to "practitioners" and "supporters" and "believers" in the subjects these sites are set up for. If you're not a fan of Apple products, don't use that site. If you think video games are a big ol' waste of time, Gaming SE isn't going to hold your interest… and if you are not earnestly interested in the precepts being discussed here, without pretense, this site may not be for you.