As a rabbi once accused of being “too soft on the Catholic Church”—liking Catholicism too much to make that particular Lutheran comfortable—I read with special sensitivity the report on a young girl and her family who left the Catholic Church for a liturgical reason, of all things.

According to the Associated Press, the young girl suffers from celiac disease, which causes her to get sick from eating gluten, a protein in wheat and other grains. She can safely eat rice. But Church law requires the Host to be made of wheat, so the family left the Church and went Methodist “where the rules on communion are more flexible because Methodists believe the bread and wine are symbolic, not the actual transubstantiated body and blood of Jesus.” That’s how the AP reported the case.

The Church’s position is explained by a spokesman for the Boston archdiocese in this language: “Bread is central to the Eucharist because of the imagery of Scripture, because of the prayers of the Christian community going back thousands of years.” The Vatican takes the matter seriously enough that, in 1994, it issued rules for all bishops to follow. Among them: “Special hosts (which do not contain gluten) are invalid matter for the celebration of the Eucharist.” And, the AP continued, citing the parish priest involved in the case,

“I think part of the problem is we are so accustomed to all these little round, pre-cut hosts we’ve lost any real sense we’re taking part in one loaf,” says the Rev. Austin Fleming, pastor of Our Lady of Christian Help Church in Concord. “We many are sharing one bread and becoming one with Christ. We can’t make different flavors for different folks and maintain that theological reality.”

Far be it from a rabbi to intervene in Catholic canon law and liturgy, but there is another explanation to consider. First, so the Church has always maintained, the Eucharist derives from the Last Supper, and the Last Supper corresponds to a Passover Seder. Now, as a matter of fact, matzah (the unleavened bread of the Passover Seder) can be prepared only from wheat or kindred grains of the same classification. To fulfill one’s obligation to eat unleavened bread, the bread must be capable of leavening but not leavened, so Mishnah-tractate Pesahim 2:5: “These are types of grains through bread made from which a person fulfills his obligation to eat unleavened bread on Passover: (1) wheat, (2) barley, (3) spelt, (4) rye, and (5) oats.” What the five grains have in common is that all ferment.

That doesn’t tell the whole story. For what other activity is a grain that ferments required? The dough offering, taken from dough prepared from the same five species of grain, must be given while the dough is being prepared; if the dough is made from other species, however, a dough offering is not separated from the dough —it is exempt. What about rice? It is explicitly excluded, according to Mishnah-tractate Hallah 1:4: “The following are exempt from dough-offering: rice, sorghum, poppy, sesame, and pulse.” What these have in common is obvious: They do not respond to the enzymes that engender life in the mixture of water and the flour of wheat, barley, spelt, oats, or rye. To this, Hallah 3:7 adds.

One who makes dough from a mixture of wheat flour and rice flour—if. . . the dough has the taste of cereal wheat, it is subject to dough offering. And a person fulfills his obligation to eat unleavened bread on Passover by means of eating it. But if it does not have the taste of cereal, it is not subject to dough offering. And a person does not fulfil his obligation to eat unleavened bread of Passover by means of eating it.

Wheat flour differs from rice in the fundamental way already noted; Wheat sustains the natural processes by which life commences and is sustained. Fermentation starts when water is mixed with flour and yeast and ends when the dough forms a crust, the enzyme in the yeast having died in the heat of the baking. The Halakhah emerges at the end of a long process of profound thought on the nature of life processes. Its message is simple: When the enzyme is activated with the addition of water to the flour and the fermentation process has begun, the obligation to separate a dough offering is incurred and must be met at the end, when the enzyme dies and the fermentation process concludes. I cannot think of a more vivid way of linking the obligation to separate a dough offering to the fermentation process. Such a process must be possible, and it must be under way. Then the consideration of God’s share in the dough registers. Or to put it somewhat materially, when the flour is brought to life by water and yeast, then God’s claim on the bread registers. The dough, when alive and expanding, encompasses a share belonging to God, which is to be removed and given to the priest.

What all this has to do with making the Host of wheat should be obvious, and it is not mere custom but theological truth. If I had to explain on behalf of the archdiocese of Boston why the Host must be made of wheat flour, I would not stop with appeals to Scripture and prayer. These are true but too general, and they are not specific to the case.

Rather, I would seize upon the substance of the issue, which consists, in my opinion, of two parts: First, the Host is explicitly unleavened bread, because that is how Jesus instituted the Eucharist at the Last Supper. The definitive trait of unleavened bread, broken “in memory of Me,” is that it derives from wheat, which can be leavened but has not been.

Second, the Host is made of wheat because wheat ferments and contains the mystery of life, represented by the working of the enzyme on the flour and water. From there, the lesson follows: the Host is the source of life not in a symbolic way but in a real one, as the Catholic Church has always maintained. The Halakhah of Judaism (in its context and for its reasons) concurs in the logic that requires the dough offering given to the priests and the matzah eaten at the Passover Seder to be made of a grain that participates in the processes of fermentation—that is to say, life.