There is a hadeeth that gives out the resolution of this issue. I am paraphrasing but its says that if two obviously Muslim people calling each other Kaafir, then one of them is Kaafir. I found this hadeeth always so beautiful and full of wisdom.
Me calling someone kaafir is not going to make that person kaafir.
and to answer your other comment, here is a quote form a Shia website
Kafir means non-believers, there are two types of Kafirs...one are Ahle-Kitaab which have been deemed as clean by most of our scholars today. The other type of kafirs are polytheist/atheists, which have been deemed as najis
i dont care what came from what shia website, calling someone kaffir like that is ridiculous
and yeah, i dunn wanna say that cuz god knows if when that happens I am the one. know what I mean mate?