He wasn't a Muslim, a Hindu, a Buddhist or a Jew. Therefore, he's a Christian by default.
It's like how I was Jewish by default when I lived in New York City: I wasn't Irish, Italian, Black or Hispanic. Us Welsh-Cherokees are rara avises.
My wife shares your Welsh/Cherokee connection. I’m Welsh/Irish/Scot with a few drops of some other invading group in southwest England/Wales 700+ years back.