I don't think it is rude, but why do you think you should attend the Catholic Mass in the first place? What good does it do to you OR to your relatives?
My biological mother and my brother are Roman Catholics, not that I would enjoy being with them, much less celebrate some family achievement at their Church. I would just do it for my father, who's a liberal Muslim, but who still wants us to be somewhat of a family...
As someone who has had to listen to Roman Catholics seeking shelter from their local happyclappydippy post V2 Latin church literally shout "AND THE SON!" during the singing of the Creed in a filioque-less ByzCath church, it would be quite rude, disruptive and just plain spiteful. Such an attitude has no place in any Christian church especially from an invited guest.
It would be far more appropriate for you to not even pray along and omit "..and the Son" because the Creed in ICEL English is presently worded in the first person plural ("We believe...") as opposed to the original first person singular.
LMAO @ the first part!
I don't know if I agree with the latter part. I wouldn't be shouting as if I were at a baseball game, but at the same time, why should I leave my beliefs at the door, even though I'm not yet an officially recognized Orthodox?
I attended a Protestant college, and at the beginning of one class we were required to recite the Lord's Prayer. I simply stopped after "deliver us from the evil one."
That's funny those Protestants would change the Lord's Prayer as it was given to us by Jesus Himself, seeing as how many Protestants are sola scriptura