The Holy Scriptures simply teach that church fellowship is altar fellowship."The cup of blessing which we bless is the koinoniaof the blood of Christ, the bread which we break is the koinonia of the body of Christ." (1 Corinthians 10:16) The Apostle connects this participation in the body and blood of the Lord immediately with the assertion that, as the bread is one, so we who are many are one body, because we partake of one bread. (v. 17) The Corpus Christi sacramentale and the Corpus Christi spirituali sive mysticum [The sacramental body of Christ and the spiritual or mystical body of Christ] as our dogmaticians say, belong essentially together. Ecclesia, "church" in the strict sense of the New Testament is there where the people of God come together at one place and celebrate the Lord's Supper. There the body of Christ in the double sense is reality, though it is of course not only there. From this view of the New Testament, that altar fellowship is church fellowship and church fellowship is altar fellowship, it follows that the boundaries of both coincide. Where does the boundary of altar fellowship in the New Testament lie? It is significant that all our documents concerning the oldest Christian Supper, insofar-as they bear a liturgical character, describe a boundary for altar fellowship. "The doors! The doors!" cries the deacon before the Creed yet today in the liturgy of Eastern Church. With this the liturgy of the "believers" begins, reminiscent of the first Sunday of the church, when the Lord came to His own behind closed doors (John 20:19). "No catechumen, no hearer, no unbeliever, no heterodox" shall be present at the Supper according to the liturgical cry of the Antiochene liturgy in the eighth book of The Apostolic Constitutions (ch. 12), and among the believers no one should have anything against another, nor should a hypocrite approach (Compare the text of Brightman, Liturgies Eastern and Western, I, p. 13). "Santa sanctis," "Holy things for holy ones" sounded the warning call before the communion. And so that no one thereby understood that the church was a union of pharisees, the response of the holy people of God sounded: "One is holy, One is the Lord, Jesus Christ, to the honor of God the Father." (Compare Brightman p. 24 et passim.) The fact that all liturgies of the old Greek Church contain such a cry by which a fence was placed around the Supper points to the fact that this is a very ancient practice. The way in which Justin [ca. 100-ca. 165] (Apology. I, 66) in his account concerning the origin of the Supper emphasizes that Jesus at the institution of the Supper gave bread and wine to the disciples only—who else could he have given it to?—shows that the "to them alone" is essential to his understanding of the Supper. The admonitions and warnings of the Teaching of the Twelve Apostles [Didache] corresponds to this. "No one is to eat or drink from your eucharist unless they are baptized in the name of the Lord. For concerning this the Lord has said: Do no give that which is holy to dogs." (Didache 9.5) Thus follows the "rubric" in the liturgy, "He who is holy, come; he who is not, repent" (10.6). This same writing prescribes confession and absolution before the Sunday celebration of the Supper in the same way the later liturgies and church orders do:
Here follows the citation from Malachi 1:11 and 14, which in this passage for the first time is applied to the Supper, though not yet in the sense of the later theory of the sacrifice of the mass. For the "sacrifice" is here still the sacrifice of praise and thanksgiving in the Biblical sense, applied to the "Eucharistia" (Didache 14). When we look at the New Testament in this light then we see immediately several passages containing the early Christian concept of the "closed Supper," namely that the Lord's Supper is celebrated behind closed doors, to the exclusion of those who do not belong at it.
First, it is certain that wherever in the New Testament there is the demand for the holy kiss (Romans 16:16; 1 Corinthians 16:20; 2 Corinthians 13:12; 1 Thessalonians 5:26; 1 Peter 5:14), the "kiss of peace," the later "Pax" which preceded the communion, is in view. The demands for this kiss occur as they do at the conclusion of these letters of Paul because they were read before the gathered ecclesia which then proceeded to celebrate the Supper. Thus the letters conclude with the "Apostolic Blessing" in its simple form, "The grace of our Lord Jesus Christ be with you," or in the developed Trinitarian form such as we find in 2 Corinthians 13:13. Is it mere coincidence that in the Greek (the liturgy of Chrysostom) and in the Syrian (e.g. in the liturgy of Theodor of Mopsuestia) Churches they do not begin the preface with "The Lord be with you" but with the formula of greeting from 2 Corinthians 13:13? The conclusion of the book of Revelation should also be compared with the Pauline letters. Is it merely coincidental that the "Maranatha! The grace of the Lord Jesus Christ be with you!" of 1 Corinthians 16:23 is repeated in Revelation. 22:20 with the words: "Come Lord Jesus! The grace of the Lord Jesus be with you all"? Was not Revelation written to be read in the liturgy (1:11; 22:18) as much as the letters of Paul? Even if it is not possible for us to know all the details of the liturgy of the first century (Pliny gives us the responsories for the time immediately before the turn of the century; the Sanctus is verified for the first century through Clement of Rome [Bishop of Rome 92-101]) the letters of Paul certainly show us this much: besides the words of institution, which belong to the celebration of the Lord's Supper, there is the demand for the kiss of peace; and then follows immediately the warning against schismatics and heretics, the anathema (Romans 16:16f.; 1 Corinthians 16:20, 22); then the ancient petition of the congregation for the coming of the Lord (still spoken in Aramaic in the Pauline congregations); and finally the benediction. The similarity of the letters of Paul with Revelation and the Didache show that these were fixed liturgical usages.
What interests us here is the close connection between the "Pax" and the "Anathema"; the kiss of love and peace, which expressed the unity and fellowship of the church, and the inflexible exclusion of schismatics and heretics from the Supper and thereby the church. At the conclusion of First Corinthians, which is directed against the divisions in the church of Corinth, it is the stubborn schismatics to whom the Anathema is directed: "If anyone does not love the Lord, let him be Anathema" (16:22). For the one who arrogantly splits the congregation, which is the body of the Lord, cannot love the Lord. In the Letter to the Romans the admonition to greet one another in peace with the kiss of love, and the assurance that the church of Rome is in this kiss bound together with all churches of Christ, is followed by the express warning over against heretics:
The fellowship of the church, the deepest and most intimate fellowship which there is, presupposes an inflexible separation from heresy (1 John 4:1–7; 2 John 9ff.; 2 Corinthians 6:14) because it is at the same time both fellowship between believers and fellowship with the Triune God (1 John 1:3). And this separation finds its essential expression in who does and who does not receive the Supper (Abendmahlszucht). The fundamental axiom of canon law that there can be no communicatio in sacris cum haereticis [lit: no fellowship in holy things with heretics] comes directly from the early church and has its dogmatic basis in the New Testament.
So also CFW Walther and the LCMS. "Members of heterodox fellowships are not excommunicated by their nonadmission to the celebration of Holy Communion in fellowship with the Lutheran church, much less are they (declared to be heretics) and condemned, but only suspended until they have reconciled with the orthodox church by leaving the false fellowship in which they stand." Theses on Communion Fellowship (1870) in C.F.W. Walther, Essays for the Church, St. Louis: CPH, p. 225. MH
 English text cited from "The Teaching of the Twelve Apostles" in The Ante-Nicene Fathers, edited by Alexander Roberts and James Donaldson (Reprinted by Eerdmans, 1979), Vol VII, p. 381 MH