The use of the word Kaffir or Kuffar can only be done so against the non-believers i.e. idolators.
Jews and Christians should never be called this.
Well, let's not start with the question whether Jews or Christians deserve to be called Kaffir.
Let's start with the question whether Muslims have any authority to even open their mouth in regards to anything.
And they don't. They were pagans before Muhammad forcefully converted them and they became Muslim pagans for following a false cult.
From the position of the first authority, I state, I do not accept Islam as a religion, and thus all of its followers are automatically kaffirs.
I did not come to them for their approval, they came to me with false claims, and I rejected their claims without prejudice.
Shalom