In short, because the Pope says so.
Regardless of what your dietary restrictions stipulate, the body of Christ is decidedly not gluten-free. That's according to a letter issued over the weekend by the Vatican at the request of Pope Francis.
The circular letter issued to Bishops specifically discussed the bread and wine used for the Eucharist. Apparently this recent reminder on what is and isn’t allowed for consumption during communion has become necessary now these items are no longer only produced within religious communities but “are also sold in supermarkets and other stores and even over the internet.” Yes, if you’re buying your communion wafers on the dark web, this could mean trouble for you.
In addressing the gluten-free crowd, the letter cites two passages within the already established rules. “The bread used in the celebration of the Most Holy Eucharistic Sacrifice must be unleavened, purely of wheat, and recently made so that there is no danger of decomposition,” the official guidelines stipulate as quoted in the Vatican’s letter. “It follows therefore that bread made from another substance, even if it is grain, or if it is mixed with another substance different from wheat to such an extent that it would not commonly be considered wheat bread, does not constitute valid matter for confecting the Sacrifice and the Eucharistic Sacrament.”
This isn’t to say that the Catholic Church is completely insensitive to the issues of those suffering from celiac disease. Instead, the letter also points out that those rules, too, have already been established. Quoting guidelines that were originally published in 2003 for those who, “for varying and grave reasons, cannot consumer bread made in the usual manner,” the letter reiterates that passage as well: “Hosts that are completely gluten-free are invalid matter for the celebration of the Eucharist. Low-gluten hosts (partially gluten-free) are valid matter, provided they contain a sufficient amount of gluten to obtain the confection of bread without the addition of foreign materials and without the use of procedures that would alter the nature of bread.”
So though the contents of this new letter may raise some eyebrows, it’s worth remembering that none of the rules appear to be new. For all intents and purposes, the Pope is simply reminding people, “If you’re going to be buying stuff off the internet, make sure it’s legit.” It’s good advice in general, really.