There are a lot of Christians, but until the twenty-first century (specifically the 9/11 attacks) they were all separate churches and the first to say so. All the mainline and fundamentalist churches insisted they were the one true faith and extra ecclesiam nulla salus, no salvation outside the Church.

Then the attacks happened, and all the faiths learned that Islam was a greater threat than each other. Also, soon after, the New Atheism movement surfaced, organized through the nascent internet, and so the fundamentalist pastors, when they spoke of conservative values and politics (which was still naughty at the time), they would pretend to speak for all of Christendom, even when the mainline faiths did not concur (and the liberal faiths, in their dwindling numbers, staunchly disagreed.)

Nowadays in the US, fundamentalist Christianity is so loud, it has drowned out all others, and white Christian nationalism (which is a subset that includes some of multiple denominations) is the loudest and most arrogant, to the point of hubris, with major figures even challenging the authority of the Holy See.