This question already has an answer here:
Does Roman Catholicism allow certain Protestants to receive communion during the mass if they have been baptized and confirmed? I'd like to know if there are any Protestant denominations that Roman Catholicism would "accept" for this purpose, such as the Episcopal Church. Does it matter if the Protestant believes Catholic doctrine regarding the nature of the Eucharist?
Or do all Protestants have to convert to Catholicism in order to be able to be permitted to participate in the mass?