Well, because it is a bad thing lol.
Someone who believes in Christ is a christian and a nationalist is someone who has immense pride for their country, often thinking that their country is superior to every other country (Nazi Germany for example). Christian Nationalism, however, is an ideology that thinks America is based entirely on Christianity and therefore Americans must be christian.
While many founding fathers were christian, they created the first amendment which included the freedom of religion and the separation of church and state (Free Exercise Clause where the government is not allowed to intervene with religious or non-religious beliefs).
But Christian Nationalism differs from Christianity because it focuses more on power and control. For instance, evangelical white supremacists during the reconstruction era called the Civil War a holy struggle/test of fate, helping southerners frame a narrative that their actions were justified and eventually forming a white supremacist social order (Jim Crow era).
I'll give you the benefit of the doubt, the definition you're likely thinking of is a Christian patriot, not a nationalist. Patriots love their country but know its limits while a nationalist would deny said limits and claim superiority. If you don't consider yourself a nationalist, congratulations idk lol