On one of the mailing lists I operate, a lady asked members to help her give some guidance to her sister who sometimes attends Lutheran services, but does not know why a Catholic should not seek to receive Holy Communion there. Here’s what I wrote:
The Holy Eucharist is at the heart of the Church. It is, in the words of the Second Vatican Council, the “source and summit” of Christian life. It’s the most important act of Christian life — to be present at Holy Mass where our Lord comes in the Sacrament of the Eucharist; and also to receive Him in the Sacrament.
The Eucharist is what gives unity to the Catholic Church. Let me explain this for a minute.
The Church exists in dioceses all over the world, and they form one fellowship, one community. They are one in faith and one in the sacraments. That is: a Catholic from one diocese is free to receive the sacraments in any other Catholic diocese. This is not something completely automatic. It happens because the bishop of your diocese and the bishops of all the other dioceses, including the Pope in Rome, all agree to be in one community together.
The theological word for this agreement is: being in communion. These bishops all agree that they are willing to celebrate Mass together, and share the sacraments together. The highest degree possible of Church unity is in celebrating the Eucharist together. So in the Catholic Church, we hold the same Catholic Faith together, and we share the seven Sacraments, including the Sacrament of Christ’s Body and Blood. Unity in Faith and unity in the Sacraments go together.
Sadly, there are divisions among Christians; there are Christians who do not hold or teach all the doctrines of the Faith that the Catholic Church holds and teaches, and they do not fully recognize the authority that God entrusts to the Catholic Church. They are not in agreement with the Catholic Church on these very important matters, so we have to acknowledge that full unity in faith does not exist yet. Without unity in faith, sacramental sharing would be a pretense: it would portray a unity that really does not exist yet.
That Christians are disunited and cannot share the Eucharist and the other sacraments is a cause of pain to us and to other Christians. We should let that pain motivate us in our efforts to bring the Christian communities together in one faith and one love, before one Holy Table of the Lord. The Church works to increase unity by engaging in dialogue with representatives of the other Christian communities and churches. When unity in faith is achieved, then full unity in sacramental sharing will follow.
Add your own arguments in the comments.