Because even if it’s just a piece of bread, it’s being offered as communion, and to receive it is to affirm the belief of the community offering it and your, well, communion with that community. Catholics cannot participate in such affirmations until unity is restored.If Catholics are sure that the bread in communion in a Protestant church is “just a piece of bread”, then what would be wrong with just eating a piece of bread? Surely eating bread is not a sin.![]()
We’re even unable to receive Communion in Orthodox churches under normal circumstances, as well as in fringe Catholic groups not in good standing with the Holy See, even through their Eucharists are valid.
Because of that, the original poster’s actions were objectively improper. Even sinful.