The Eucharist is the central sign and practice of the body of Christ. The normal way of becoming a member of that body is through baptism. To reserve Eucharist for those who are baptized does not limit God. As Luther insisted, Jesus – risen and ascended – is present everywhere and can surprise us in our cabbage soup if he so desires. While he is free to surprise anyone anywhere, the promise the Church claims is that he will not surprise us by not being present to his body, the Church, as Redeemer and Judge in the waters of baptism and in the bread and wine of the Eucharist. God is free to surprise us by working outside the norms as has surely happend with some who have participated in the Eucharist before being baptized (cf. Take This Bread by Sara Miles).
Because God remains free to surprise us there is no need for hyper-vigilance to protect the purity of the Eucharist. But if we are not to be hyper-vigilant, that doe not mean that distinctions and norms are irrelevant or unnecessary. The discipline of reserving Eucharist for those already baptized is not about protecting anyone’s purity. It is about maintaining the very boundaries of identity that make a place in which to be formed as a community that can actually practice hospitality.
It is also about being honest about who we are called to be as the members of the body of Christ, what kind of God we have gotten caught up with. It is about being respectful of the real otherness of those who are not yet committed to the loyalties and commitments of this communion.
The body of Christ is a Eucharistic community with all that that entails and we are baptized into Eucharist.