The Holy Eucharist is the central event in the life of the Church. Through it the faithful become partakers of the Body and Blood of Christ (Mat. 26, 26-28; Mark 14,22-24; Luke 22, 15-20; Jn 6, 51-56; I Cor. 11, 24-26).
How can Christ offer us His Body to eat and His Blood to drink? This was also a question which the Jews raised, and even some who followed Christ and were His disciples. Christ, however, insisted that they must do so, and explained that He was not referring to dead flesh but to His Body, which was united with the Holy Spirit, which vivifies (John 6, 52; 60- 63). In the Holy Eucharist bread and wine are offered and God accepts this oblation of man. He changes these elements and in turn offers them to man as His Body and Blood, as participation in the sacrifice which Christ offered on Golgotha "once and for all". (Heb. 7,27; 9,12,28). Before His sacrifice on the Cross, Christ celebrated this "Supper" and commanded His disciples to do the same until His Second Coming, declaring that the "food" of His Body and the "drink" of His Blood were necessary for salvation (Jn 6, 31-50; I Cor. 11,23-29).
This "Supper" as "food" and "drink" of the Body and Blood of Christ is not understood apart from, and independent of, the sacrifice on Golgotha; it constitutes "participation" in this unique sacrifice. The fact that the Holy Eucharist was celebrated before Christ's sacrifice demonstrates that its identity with the sacrifice celebrated "once and for all" cannot be comprehended by human logic; it can be understood only "in mystery". The same holds true with the Holy Eucharist celebrated today within the Church.
Of course the Holy Eucharist also constitutes a remembrance of Christ's passion (Lk 22,19; I Cor. 11,24-25), but it is not only a remembrance. Already in the Old Testament it was prophesied concerning the messianic age, i.e. the Church, that "a pure sacrifice" would be celebrated (Malachi 1,11). Here is meant the sacrifice which according to the New Testament is offered on the Christian "altar" from which the Jews cannot eat (Heb. 13,10).
St. Paul proceeds to a comparison of the Christian altar (the "table of the Lord"), with that of Israel of the flesh, and that of the idolaters. He underlines that participation in the Christian altar makes Christians partakers of the Body and Blood of the Lord. On the contrary the participation in the altar of the idols makes the idolaters "partakers of demons". The "Lord's table" then, is according to St. Paul, the only true altar of the living God.
The Holy Eucharist constitutes the expression of God's great love for man. In the person of Christ He Himself sought out apostate man. Now he feeds us with His own Body and Blood, just as a mother, full of tenderness and compassion, feeds her child, not with other food, but with her very own milk, which is her very blood. He condescends to our weakness and employs basic elements from our daily fare, bread and wine, which He changes into His Body and Blood.
Through the Holy Eucharist the purpose of the divine dispensation is realized in the person of Christ, for it is the synaxis or gathering "into one" of God's scattered children (Jn 11,52), into one Body (I Cor. 10,17), and the constituting of His Church. It is for this reason that the gathering or synaxis for the celebration of the Holy Eucharist is call a gathering "in Church" [η Εκκλησία] I Cor. 11, 18) and "Kingdom of God". This is why the Liturgy begins with the phrase: "Blessed is the Kingdom of the Father, Son and Holy Spirit". A beautiful prayer of one of the early Christian liturgies expresses precisely this: "Just as this particle from the bread was as sheaves of wheat scattered on the mountains and became one bread, thus let Your Church be gathered from the ends of the earth into Your Kingdom".
The Holy Eucharist is presided over by the bishop or by a presbyter. It is not he, however, who performs the Eucharist: Christ is He "who offers and is offered"; the priests are Christ's ministers, stewards of His Mysteries (I Cor. 4,1). The laity also actively participate in what takes place; they are not passive witnesses. Liturgy means the "work or task of the people" [in Greek, λαόν έργον], that of God's entire people, not only the clergy, for the latter are included within God's people and do not stand above them.
Each member has his ministry, in accordance with the gift he has received. The laity do not have as their own the special priesthood of the clergy, but neither do the clergy understand the lay element to be passive recipients of "what is being performed." Such a distinction between "performers" and "witnesses" in the Orthodox Liturgy is unacceptable.
The active participation of the faithful in the Holy Eucharist, the communicating of the Body and Blood of Christ is essential for salvation, for by it the faithful are kept alive spiritually. This is why the Orthodox Church offers Holy Communion to infants as well, in obedience to Christ's commandment (Mat. 18,2-5. 19, 13-15). The Orthodox Christian does not consider Holy Communion to be common food. He thus properly prepares for its reception through prayer and fasting. He follows the injunction of St. Paul who assures us that Holy Communion is indeed true communion of Christ's Body and Blood, and declares: "Let each man examine himself, and thus eat of this Bread and drink from this cup" (I Cor. 10, 16-21. 11, 26-28).
But how can a believer realize that a specific Eucharistic gathering belongs in truth to the Orthodox Church? How can he discern whether or not a Divine Liturgy is in all its aspects orthodox? In this, it is not enough that there be certain external similarities, or even that the orthodox text of the Divine Liturgy is followed.
St. Ignatius of Antioch gives us the answer to this question: Orthodox is that liturgy which is performed by the bishop, or by a presbyter in communion with the bishop and has the necessary authorization. The bishop is the guarantor of Christ's presence, because he has his priesthood from Christ Himself, through Apostolic succession, which is a continuous and unbroken chain of unity preceding back to the Apostles.
It is thus necessary that the priest who performs the Holy Eucharist possess a valid ordination and be in union with the bishop of the local Church. The bishop's presence at every orthodox liturgy is manifested in that the Divine Liturgy is performed upon an antimension which bears the bishop's signature, and that during the Liturgy the bishop's name is commemorated. Indicative is the fact that the priest who performs the liturgy does not commemorate the name of the bishop who ordained him but that of the bishop in whose diocese the liturgy is being performed.