I let a girl down at Christmas. She left me a message "Remember to bring an umbrella if you believe in rain", saying it was a philosophical remark. What does this mean?

It means to keep your promise, rain is a promise or promise, and remembering to bring an umbrella is to remember to keep your promise and remember to realize it. Translated, that is to say, since you have made a promise, you also believe that your promise can be realized, so you should realize it instead of just treating it as empty talk. Did you break the promise between you?