Because eternal salvation is a lot more important than 80-or-so years on earth.
This mindset is why traditional Christianity is distasteful to me. It places evangelism over compassion for your fellow man, and it paints God as an asshole; the former is dehumanizing, the latter frankly blasphemous.
I understand where you’re coming from, but why is life more important than knowing truth? Not that all possible precautions shouldn’t be taken to prevent the loss of life before spreading the truth. Christianity rejects the modern premise that truth is either unknowable or unimportant, and because Christians believe truth to be inextricably tied to eternal life, knowing and living (or dying) according to the truth is the greatest good. And consequently, a short life with knowledge of the truth is better than a long life in ignorance of it.
I also reject the premise that truth is unknowable/unimportant. Difference between our perspectives, though, is that I believe spiritual truth can be discerned intuitively, and one doesn’t need to belong to any particular religion to discern it.
I believe that to some extent (natural law, and all that), but I don’t know if I would call it “spiritual truth.” What exactly do you mean by that, if I may ask?