It is a sin to lie about others and what they believe. I have told you the truth - it is you who are wrong.
I am not a RC scholar, so please give me some room for error if I'm wrong (and feel free to correct me), but I do see a difference in the way many Protestants interpret Scripture, versus the way RC's interpret. Protestants approach the Bible with the understanding that there are figures of speech "metaphors" and "similes"With. These are used in our present day language as well. When these moments arise, we intuitively know that everything is based on context. With this in mind, we know that when we approach passages that appear to have one meaning, but our general sense of "reality" says otherwise, we must look for the spiritual context of the passage. The passage of the Lord's supper is a perfect example...based on the general revelation God has given us (common knowledge via our senses evidenced by sensual experience) we know the difference between wine and blood... a wafer and flesh. Okay, so my point is this, at some point, the average RC member must stop and question the teaching of its leadership...we Protestants do this all the time with our leadership, because guess what? they are often wrong on serious theological doctrines. So I would just encourage all RC's to not follow for the sake of tradition or comfort, you can be sincere, but you can also be sincerely wrong.