You choose your Patron Saint, either based on your name - your birth name or your confirmation name - or for some other reason. For example, my Confirmation patron is the Empress St. Helena, mother of the Emperor Constantine. However, I also consider St. Martha (grumpy housekeeper) and St. Matthew the Tax-collector patrons.
As for your Guardian Angel, that’s sort of intuition. Mother Angelica suggests addressing your Guardian Angel by a name, while recognizing that this is a convenience, rather than having identified a particular angel among the myriads who exists. Mine is “Amparo,” which means “helper” in Spanish.
Our new priest told me the other day. That is was no mistake that I got that name.
I always thought she got a bum deal with that reputation. She was one of the first to acknowledge Christ as the Messiah.
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I love that. :)