If you want the clearest and most succinct explanation of why orthodox Lutherans declare church fellowship before they share in altar fellowship, read Hermann Sasse’s “Theses on the Question of Church and Altar Fellowship” in The Lonely Way, vol. 1. Sasse distills down to their essence the Scriptural principles of unity and confession by which the Church lives in this world.
There is only one Christ, so there can only be one Body of Christ, the Church. But the “unity of the church, and with it the indissolubly connected unity of the Supper, will not be seen by the world. For since the days of the NT, where people believe in Christ and desire to belong to his church, they have been rent asunder by schism and heresies into various fellowships, and there have been division and separation at the Supper” (332). Sasse emphasizes again and again in his essays that there never was a golden age of the Church, where there were no divisions and everyone agreed. This is why every attempt at restorationism, or returning to such a golden age, only ends in more division.
Sasse is not afraid to deal with the scandal caused in the world by what the world views as a destroying of the bond of love among and between Christians. But:
The worst difficulty which the splintering of Christianity has brought can never be overcome simply by declaring that the barriers between the altars are no longer present, and by pronouncing a general altar fellowship. Altar fellowship is only possible where a real church fellowship already exists. Should altar fellowship be pronounced, as was the case in the Unions of the previous [Nineteenth] century, as the means and beginning of a prevailing church fellowship, not only is this fellowship not established, but the church is also destroyed. Such measures make the church a human religious society, and the Supper is made a mere religious celebration of such a society. This thesis has been confirmed by the experiences of all unions, which treat altar fellowship not as goal, but as point of departure for ecclesiastical unification (333).
The divisions among the churches are caused, on the one hand, by the sin of “lovelessness, which lead[s] to schism and division of the congregation,” and, on the other hand, by “the intrusion of heresy into the congregation, which leads to the formation of sects and necessitates the separation of pure doctrine from false, the church from the sect” (333). Here Sasse gives examples of both from the Scriptures: on schism, e.g., 1 Cor. 1:1ff.; 11:18; Eph. 4:1ff. On heresy, e.g., 1 Tim. 4:1; 2 Tim. 3:1ff.; Gal. 1:7ff.; Titus 3:10; 1 John 4:1ff.; 2 John 10ff.
He who destroys the unity of the Christian congregation sins against Christ. He who causes divisions of the congregation about the Supper celebrates the Supper unworthily and eats and drinks the body and blood of the Lord to judgment. … [And] Christianity has the duty to strictly avoid every church and altar fellowship with heresy, to examine individual believers, to instruct the erring in love, and to most strenuously advance church and altar fellowship within orthodox Christianity.
6. The fulfillment of this duty presupposes the clear knowledge of what pure and false doctrine, what church and heresy are” (333).
He goes on to discuss both the Roman and Reformed communions, on either side of the Lutheran “lonely way.” He says that the “judgment that another church is a heretical church, with which one may not have church fellowship, in no way entails that this church must then be treated only as a synagogue of Satan, or a ‘devil’s church'” (334). So: “One may see therein an unbearable contradiction: that, to be sure, heresy comes from the devil, but that also among heretics the church of Christ may yet exist. Yet he must grant that this contradiction stretches through all of church history, from the controversy over Baptism by heretics to the struglle over the Baptism of rationalists” (334). Nevertheless, the Lutherans have never drawn from this fact (that there are truly Christians in other communions) the conclusion “that one may thus commune at Roman altars” (334); nor is it possible, though the Reformed may indeed have Christ’s Supper, “there is for the Lutheran Christian no possibility, not even in the peril of death [periculo mortis], of taking part in the Reformed Supper [nota bene ELCA!]. … Altar fellowship with the Reformed churches would only be possible if they were to deny Calvin’s doctrine and teach the bodily presence of Christ under the forms of the bread and wine” (335).
Here Sasse highlights how the Reformed and the Lutherans accuse each other of the two opposite causes of division: the Reformed accuse the Lutherans of lovelessness and schism; the Lutherans accuse the Reformed of heresy. “The situation is this: that either the Lutheran Church can surrender to the Reformed doctrine, or the Reformed Church to the Lutheran doctrine of the Supper, but there is no higher unity transcending both” (335). For the Lutherans, there can be no church or altar fellowship with the Reformed unless and until the “Reformed churches have renounced their errors” (336). And if a Reformed Christian wishes to receive the Sacrament at a Lutheran altar, he or she must confess “the doctrine of the Lutheran Catechism. … Therefore the participation of a Reformed [Christian] in a Lutheran Supper means his joining the Lutheran Church” (336).
Finally, Sasse answers the objection that his theses “can ‘no longer’ be carried out in the practice of churchly life.”
To this we must immediately answer: If strict churchly and confessional principles can “no longer” be carried out in our time, then there is no point in maintaining an Evangelical Lutheran Church. But then we would do well to ask ourselves whether the truths of the Reformation still apply. Luther did not ask how the truths of the Reformation would play out. What is really true and right is just as difficult or easy to carry out in the twentieth [or twenty-first] century as it was in the sixteenth (336, emphasis added).
“We will even have to learn,” he writes, “to improve the diminishing abilities to think through ecclesiastical questions and to come to the correct conclusions. Certainly any new arrangement will not be brought about quickly. What has been neglected for centuries cannot be made good in a few years. We must think in terms of decades” (336-337).
We know today what a perverted doctrine of the Supper and its corresponding practice has produced in our churches. It has nearly robbed us of the Sacrament and thus nearly destroyed the church. The renewal of the doctrine of the Sacrament, which we are experiencing today with astonishment, will be followed by the renewal of the correct celebration of the Holy Supper. And if this renewal is carried out first in a few places, and in smaller circles, if it is really the rightly understood and rightly celebrated Sacrament of the Altar, then the church will necessarily be renewed through it. For the church, which is the body of Christ, is built on earth when Christ feeds his community which truly believes in him with his true body and blood (337, emphasis added).