Christian is pretty vague and most people have different definitions for that term. Some consider Catholics to be Christians while others don’t. I think you are safe with either option there. Doesn’t hurt to throw a bible verse or two in there to support your beliefs. Asking for a religious letter from anyone is unreasonable and a religion can be your own, personal belief and not include any organized movement. Peggy Hall has some great videos on YT that could help you too.