The Sacrament of Holy Eucharist is the God-ordained mysterion in which Christ Himself "is" in reality in the Elements of Bread and Wine. Holy Eucharist is offered for the communion of the believers with God;it is a real and bloodless act of Christ's sacrifice on the Cross. This mysterion embodies both the Sacrament and the Sacrificeof Christ.
Christ instituted this sacrament as the everlasting Sacrament of the New Testament. The Eucharist was ordained to be a Sacrament offered continuously and everlasting. In the other sacraments Christ the Saviour sanctifies the believers through His Grace, bt in the Eucharist Himself "is" the hypostasis and substance.
The Apostles and the Church from the beginning interpreted the words of Christ at the institution of the Eucharist as meaning that Christ was really in the Eucharist. The Eucharist was not an imitation of the Jewish Passover.
In reality the Elements of Bread and Wine become in substance the very Body and Blood of Christ. The words of Christ signify the actual "change" rather than the co-existance of visible and invisible parts.
The institution and the transmission of the Holy Eucharist in the Elements of the Bread and Wine signify the offering of the Sacrifice of Christ on the Cross. Thus the Body and Blood of Christ exist in the unty of the elements, not separately. Since Christ is ever present in the Church the House of God is only used for prayer and worship, and not as a place for assemblies, etc.
The Epiklesis (invocation of the Holy Spirit) fr the sanctification of levened Bread and Wine with water is the essence of the Divine Liturgy. The use of leavened bread was practiced by the Church at the very begining. The loftiest, most sacred moment of the Divine Liturgy is when the invocation of the Holy Spirit takes place to "change" the elements.
The officiator is a priest or bishop. The partakers of the Holy Eucharist are baptized members of the Church, including infants.