Protopresbyter Peter Heers D. Th.
“This is a fundamental identity: the Church in the Eucharist and the Eucharist in the Church. Where the God-man is not, the Church is not, and where the Church is not, there is no Eucharist. Everything outside this is heresy, non-church, anti-church, and psuedo-church.” St. Justin Popovich
The identification of the Body applies to the Body as a whole and to each of its aspects simultaneously. Each manifestation of the Body contains within it the fullness of the Body. “Each mystery constitutes a particular aspect or manifestation of a united reality,”  of the one mystery of Christ (Eph. 3: 4), “which hath been hid from ages and from generations, but now is made manifest to his saints” (Col. 1: 26). We recognize a mystery, such as holy Baptism, only when it is a reflection of the One Church. “No mystery can be conceived of per se, but only in relation to the Mystery, which recapitulates the entire ‘mystery of Christ,’ that is, the Divine Eucharist.” 
The Baptism of the Church is not simply form, matter, and intention. It is first of all initiation. That Baptism that we recognize as the one Baptism brings one into the life of the Church, the heart of which is the Eucharist. As Fr. George Florovsky has written: “The entire meaning and strength of the sacrament of Holy Baptism is that the baptized enters into the one Church, ‘the one Church of angels and men,’  taking root and growing into the one Body of Christ, and becomes a ‘fellow citizen of the saints and friend of God’ [Eph. 2: 19], for ‘we are all one Spirit baptized into one body’ [1 Cor. 12: 13]. Holy Baptism is a mysterious initiation into the Church, as into the kingdom of grace.” 
Hence, if one is not initiated into the Church, if one does not enter into the one Church, into a particular local church through his parish community,  and become a member of the Body by partaking of the Eucharist, it would be impossible for the Orthodox to recognize that he has been truly baptized. Such a Baptism is not the Baptism of and into the Church. Such a Baptism, “a Baptism disconnected from the Holy Eucharist,” “is a death without resurrection.”  How does Baptism integrate us into the Church? Precisely by opening us up to the gift of the the Holy Spirit, which then gives us access to the Eucharist. The one presupposes the other, for they all belong together, with the Eucharist being the “self-evident fulfillment” of the others.  There is a “sacramental interdependence” such that it is impossible to speak of one without the other two, impossible to speak of someone being baptized without approaching Christ’s table in His Kingdom. In the words of Father Alexander Schmemann, for the Fathers the Eucharist is “the ‘focus,’ the source and the fulfillment of the entire— and not merely the liturgical— life of the Church, the sacrament of the Church’s self-manifestation and edification.” 
Baptism as integration, as entry, presupposes communion in the common cup of the Eucharist. For, “if the Church’s ultimate being and essence are revealed in and through the Eucharist, if Eucharist is truly the sacrament of the Church and not only one of the Church’s sacraments, then of necessity to enter the Church is to enter into the Eucharist, then Eucharist is indeed the fulfillment of Baptism.”  No mystery is an end in itself— except for the Eucharist. All other mysteries must be placed in the context of the Eucharist. Therefore, the faithful are baptized “so that having died with Christ they might partake of His Risen Life, and it is this Risen Life that the Eucharist manifests and communicates in the Church, making her members into witnesses of the things to come.” 
Having this in mind, when we turn to the text of Unitatis Redintegratio 3a, which recognizes those among the “separated brethren” who are not in “full communion” with the Roman Catholic Church as being “truly baptized” and “incorporated into Christ,” members of Christ’s Church, one is at a loss to know what this could mean. What kind of Baptism is this that incorporates into Christ without leading to the fulfillment of Baptism in the Eucharist? Or, what kind of “incorporation” is this that is effected without the Eucharist, since becoming one with the Body of Christ takes place in the Eucharist?  For what else could “incomplete communion” mean here except that they have not reached the “summit” of communion, according to Cardinal Kasper’s description of the Eucharist? Certainly, as it pertains to most Protestants who do not have a “valid” Eucharist, this must be what is meant. Thus, it is evident that what the mysteries, Baptism and the Eucharist, are understood to mean by the Orthodox does not coincide with the meaning found in Unitatis Redintegratio and Lumen Gentium.
The implications for ecclesiology are immense, for the members of the Church are constituted as the Church first and foremost through these mysteries. The separation and independence of Baptism from the Eucharist, on both a theoretical as well as a practical level, is not only unchallenged in Unitatis Redintegratio, it is an important pillar of the new ecclesiology developed therein.  This independence of Baptism from the Eucharist signifies much more than simply a liturgical diversion from traditional practice. It touches upon the faith itself and signals “a deep perversion of the identity of the Church with wide-ranging and serious consequences.” 
One cannot be incorporated into Christ and become His member in Baptism alone.  The Church is not created in the waters of Baptism alone, but, rather, was born from the side of Christ when “forthwith came there out blood and water” (John 19: 34); neither blood alone, nor water alone, but both together.  Those born have to be nourished; those baptized partake straightaway of divine food. That is why, for the Orthodox, “every Eucharistic assembly is an assembly of the entire Church,”  τὸ πλήρωμα, “the flesh of the Church”  which Christ assumed. Those not incorporated into this assembly  are not of the fullness, which means they have not been made members of Christ’s Body. For, we know of no such Baptism that is not fulfilled in the Eucharist. (The Ecclesiological Renovation of Vatican II: An Orthodox Examination of Rome’s Ecumenical Theology Regarding Baptism and the Church. 2015-11-16. [Kindle Locations 2597-2649]. Uncut Mountain Press. Kindle Edition)
. John Zizioulas (Metropolitan of Pergamon), “Holy Baptism and Divine Liturgy,” in Holy Baptism: Our Incorporation into the Church of Christ [in Greek: “Ἅγιον Βάπτισμα καὶ Θεία Λειτουργία” στό Τό Ἅγιο Βάπτισμα: Ἠ ἔνταξή μας στήν Ἐκκλησία τοῦ Χριστοῦ (Athens: Apostoliki Diakonia, 2002), 11].
. Zizioulas, ibid., 12.
. Paraklitiki (Divine service book), Tone 1, Wednesday morning, aposticha.
. Florovsky, “House of the Father,” 79.
. “The fact that the newly illumined one must immediately gather epi to auto, and not simply commune of the Mysteries, means that with Baptism and Chrismation he is inscribed into a particular local church through his parish community, and is under a particular bishop, who presides at the Eucharist. Just as there are no absolute ordinations, neither can there exist absolute Baptisms.” (Zizioulas, “Holy Baptism and Divine Liturgy,” 24).
. Ibid., 20.
. The patristic witness to this unity of the Mysteries of Baptism and the Eucharist is ancient. See, for example: Saint Justin the Philosopher, First Apology, LXV; Psuedo-Clement, 100, 141; Hippolytus of Rome, Apostolic Tradition, 21; Canons of Hippolytus, 21, § 142– 143; Saint Ambrose of Milan, On the Mysteries, 8; Saint John Chrysostom, Catechetical Homily II. 2 and IV. 6; Saint Basil the Great, Concerning Baptism, 1.3. See the brief treatment of these sources and their witness in John (Zizioulas), Metropolitan of Pergamon (“ Holy Baptism and Divine Liturgy,” 13– 15. For a more extensive treatment of these sources see I. Yazigi (Hani), Hierodeacon, Ἠ τελετή τοῦ ἁγίου βαπτίσματος, (Ἱστορική, θεολογική καί τελετουργική θεώρησις) [The service of holy Baptism: Historic, theological and liturgical consideration], doctoral thesis, Thessaloniki, 1982).
. Schmemann, Of Water and the Spirit, 117.
. Ibid., 117– 18.
. Ibid., p. 119 (emphasis added).
. Stressing that the Divine Eucharist is the perfection of all the mysteries and the image of the Kingdom of God, Met. John (Zizioulas) of Pergamon asks rhetorically: “What benefit is Baptism, when the baptized does not immediately join the Eucharistic synaxis epi to auto? Can he become a son of the Kingdom without this?” (“ Holy Baptism and Divine Liturgy,” 23).
. See UR 3 and 22. 530. Zizioulas, “Holy Baptism and Divine Liturgy,” 27.
. One cannot stress this point enough. For Orthodox Christians it is an heretical idea to believe, as Fr. John Romanides has written, that “all baptized Christians are members of the body of Christ even though they hardly go to Church to commune and have not the slightest desire to struggle for selfless love and fight against the devil epi to auto as they solemnly swore at Baptism” (“ The Ecclesiology of St. Ignatius of Antioch,” The Greek Orthodox Theological Review 7: 1 and 2 [1961– 62]). “[ G]race is never given absolutely, but always in the synaxis and in the Church” (Zizioulas, “Holy Baptism and Divine Liturgy,” 26).
. See St. Cyril of Alexandria, On John, 12, PG 74.677B, and St. John Chrysostom, On John, 85.3, PG 59.463. “Let it not be forgotten that not only does the Eucharist give meaning to Baptism, but that Baptism constitutes an inexorable presupposition for the Eucharist” (Zizioulas, “Holy Baptism and Divine Liturgy,” 26).
. Hieromonk Gregorios, The Divine Liturgy: A Commentary in the Light of the Fathers (Mount Athos: Cell of St. John the Theologian, 2009), 26. “[ A]ccording to St. Dionysius the Aeropagite and the entire Patristic Tradition, at least up until St. Nicholas Cabasilas, the Eucharist alone gives meaning to every ecclesiastical and liturgical action precisely because it is a synaxis of the entire Church . . .” (Zizioulas, “Holy Baptism and Divine Liturgy,” 23).
. St. John Chrysostom, Homily before his Exile, 2, PG 52.429.
. “Without the synaxis [of the Eucharist] no liturgical action of any kind can have any meaning whatsoever, nor even the entire Christian life, I would say, including asceticism, the virtues, etc.” (Zizioulas, “Holy Baptism and Divine Liturgy,” 23).