Every Sunday we say, in the words of the Nicene Creed, that we believe four things about the Church: that it is (1) One, (2) Holy, (3) Catholic, and (4) Apostolic. We see all of these elements in our Lord’s high priestly prayer in John 17. Look at the whole thing; its incredibly rich.
Let's look briefly at apostilicity and unity in John 17.
The Church is Apostolic
Jesus was talking to the eleven apostles (Judas having left), so there we have “apostolic.” We know he was praying specifically for the Apostles because he was speaking at the Last Supper, the night before his blessed passion, and St. Matthew in relating the same events tells us who precisely was present: “When it was evening, he sat at table with the twelve…” (see Mat. 26.20). John situates the prayer for unity in our Lord’s long discourse after “…he had washed their feet, had put on his robe, and had returned to the table…” (Jn. 13.12), and there follows the four-chapter long discourse in which the prayer for unity is situated. Here too, by the way, we see the scriptural linking of the Church’s apostolicity with its sacramentality: this prayer comes at the institution of the Eucharist, at the Last Supper. Hence the multiplicity of related meanings of the word “communion” – as in “Anglican Communion” on the one hand, and “Holy Communion” on the other.
The Church is One
Our Lord prays not only for the Apostles, but for “those who believe in me through their word” (Jn. 17.20). He prays that those who believe in Jesus through the teaching of the Apostles might be one with the Apostles, and thereby one with himself, and thereby one with the Father. But this is all through the ministry of the WORD, through the Apostles’ teaching. Why? Because it is our Lord’s own teaching. And Christ’s teaching, his “word,” comes from the Father: “…I do nothing on my own authority but speak thus as the Father taught me” (Jn. 8.28). And the Lord says of the Apostles: “I have given them the words which thou gavest me, and they have received them…” (Jn. 17.8), and “they have kept thy word” (Jn. 17.6). The Lord says clearly that to hear those whom he sends is to hear him; and likewise to reject those whom he sends is to reject him: “He who hears you hears me, and he who rejects you rejects me, and he who rejects me rejects him who sent me” (Lk. 10.16). The unity of the Church is the unity of the Lord with the Apostles – “I in them and thou in me” (Jn. 17.23) – and it is therefore not to be taken for granted; it is a gift, and it is given not just to anyone, but expressly to “those who believe in me through their word” (Jn. 17.20).
The Church’s unity – its oneness – therefore comes through its share in and its reception of the words of God (the theou logoi or, loosely speaking, a unity of theology), which words the Father has given to the Son, and which the Son has given to the Apostles, and which they in turn have given to others. The Father’s gift of his Word to the Son is constitutive of the Father’s having eternally begotten the Son. That is, the Father’s gift of the Word to the Son is an eternal gift, and a gift so tightly given and so closely received, that it constitutes the very essence of God as Son. “In the beginning was the Word, and the Word was with God, and the Word was God” (Jn. 1.1). This, again, can be seen in the Nicene Creed: the Word of God is “begotten of his Father before all worlds,” “of one substance with the Father,” and “very God of very God.”
Christ’s gift of the Word of God to the Apostles is shown to be the essence of the oneness of the Church as the Body of Christ. As I have mentioned, this discourse in John is presented in the context of the institution of the Eucharist, where the incarnate Word gives HIMSELF to the Apostles: “This is my body which is given for you” (Lk. 22.19). And therefore the Church rightly recognizes the yoking of preaching the Word and ministering the sacraments: “Give grace, O heavenly Father, to all bishops and other ministers, that they may… set forth thy true and lively Word, and rightly and duly administer thy holy Sacraments” (BCP p. 329). In full expressions of the Church, therefore, it is recognized that to proclaim the Word of God is to imitate Christ in his offering himself to the Father, because the Word of God is not just the abstract teaching of the Apostles, and not just the Bible, but rather as John 1.14 says “the Word became flesh.” If to preach the Gospel is to proclaim the Word of God (and it is), then it is not merely to proclaim a teaching (it is that; but its not just that), but it is even more fundamentally to offer the flesh of Jesus Christ. The whole reason for preaching, for proclaiming the Word, is because it is the enterprise of holding up the unique (unique → unity), which is to say the one flesh of Jesus Christ who is the Word of God: “‘and I, when I am lifted up from the earth, will draw all men to myself.’ He said this to show by what death he was to die” (Jn. 12.32). When Jesus speaks of his being "lifted up" he is speaking, in essence, of his proclamation of himself as the Word of God.
And this unity is the essence of the Eucharistic sacrifice. It is a perpetuation of the Apostolic power of offering the Word of God, which has become unique flesh. To offer the Word is therefore to offer a spotless and immaculate victim, the flesh of the man Jesus of Nazareth, who is of one substance with God the Father. Preaching the gospel and offering the Eucharistic sacrifice are forever and inextricably linked precisely because God’s perfect offering of his own life to humankind is forever and inextricably linked to the offering of perfect human nature to the Father in Christ’s “one oblation of himself, once offered” on the cross. In the crucified flesh of Jesus Christ there is at last perfect intercourse between God and man – a perfect, loving, simultaneous, and mutual outpouring of natures – because it is the ONE Christ who is crucified, and “although he be God and man, yet he is not two, but one Christ” (as the Athanasian Creed affirms). Christ’s sacrifice is the loving and simultaneous self-offering of God to man, and of man to God.
But it is realized immanently only by those whose faith in Christ is circumscribed by the teaching of the Apostles, viz. "those who believe in me through their word." Is this ECUSA?