I'm struggling to find this idiom/phrase that I've heard before that goes along the lines of "I'll die by _____" in the sense of I'll always believe that that is the absolute truth, and no one will convince me otherwise until the end of my days. But googling that exact phrase only gives me 2 results, so I know that I'm must be mistaking it with something else.
So is there any idiom that uses "die" in the context of an opinionated person that refuses to change their minds? Or is there a similar phrase for this?