well, yes. We’re arguing with what we thought were a bunch of Protestants, but now it seems that they are not, as they do not even believe in the basic Christian concepts like the Trinity
Those who would reject the existence on one holy catholic and apostolic (which, for me is found in the Roman Catholic and Eastern Orthodox Churches) find themselves in a tight spot when pinned down abou the basics.
At some level they know they need to align with the Christologic definitions that the councils delivered. At least, for me, when I actually read the councils, they proclaimed what I had been taught about who Jesus was (and was not).
I had no clue, until I began studying, that the questions were settled hundreds of years ago and that the answers were what I already believed. Of course, by then, I was honestly and passionately seeking - not defending a position.