It's a phrase that sounds horrible. I certainly would tend to believe that it's a bad thing, based only on the unexplained phrase. But I would check the box that says, "What the f**k are you talking about?" because I am an intelligent, rational, thinking human being and this is the first time I have ever heard the phrase "Christian Nationalism."
It's hardly in the mainstream conversation. I Googled it under "news" and found references in Morning Consult, LGBTQ Nation, Rewire News, Christian Post, AlterNet, High Plains Reader and, of course, Politico.
The average voter would check the "critical threat" box or the "important threat" box, because the poll includes two choices to label it a threat and only one choice to label it not a threat, and the two choices labeling it a threat come first. The non-threat choice is worded in a highly certain manner, admitting to no uncertainty in the answer, and the poll does not, of course, define what "Christian Nationalism" is, but repeats the phrase twice. That's how pollsters create polls to generate the results that are desired by the people who are paying them to create the polls.
Edit: I should say it's certainly not big in the mainstream conversation. Can't say it's not there at all, but...