It is a sect of Christianity, but a fair amount of protestants don't like to acknowledge that. I grew up in a very Presbyterian family, and I straight up didn't know that Catholics were Christians until I was an adult because I was always taught that they were sinners who were obsessed with meaningless rituals, who worshipped the virgin Mary and saints rather than god (i.e. "false idols"), etc.

I'm no longer religious, but my family still is and my mom - to this day - refuses to acknowledge the fact that Catholics are Christians.