"how can anyone at all deny that christ died for us?"
I have many reasons as to why I don't believe in a God/Christ, or at least I'm uncertain as to what created us, how or why? I believe that makes me an Agnostic.
back to your statement. let's just say my childhood was less than ideal, and by that I mean I suffered a fair amount of many forms of abuse. You can call BS becuase you don't know me, but I assure you I have no reason to lie. In my youth, in many tough situations, I would ask "How can anyone at all believe that christ existed? How could god let all the bad things happen to me and millions of others around the world?"
So I say, it's very easy for people to deny that christ died for us based on personal experience or differing religious beliefs.
You're a passioante person, and your love for God is good, for you. When you draw a line in the sand however and make a statement as such, try and think about how people with other views can't relate to a comment like that. And hopefully you don't look down on them for their beliefs, which ideally is reciprocated.