The Eucharist completes Christian initiation. Through this sacrament of initiation we participate with the whole community of believers in the Lord's own sacrifice. The Eucharist (from the Greek word eucharistia 'thanksgiving') is the sacrament of the body and blood of Christ in which he is present under the forms of bread and wine offering himself in the sacrifice of the Mass and giving himself as spiritual food to the faithful.
At the Last Supper, the Lord instituted the Eucharistic sacrifice of his body and blood. He did this in order to perpetuate the sacrifice of the cross throughout the ages until he should come again, and so to entrust to the church a memorial of his death and resurrection. The Eucharist is the source and summit of the Christian life.
Guidelines Taken from USCCB
As Catholics, we fully participate in the celebration of the Eucharist when we receive Holy Communion. We are encouraged to receive Communion devoutly and frequently. In order to be properly disposed to receive Communion, participants should not be conscious of grave sin and normally should have fasted for one hour. A person who is conscious of grave sin is not to receive the Body and Blood of the Lord without prior sacramental confession except for a grave reason where there is no opportunity for confession. In this case, the person is to be mindful of the obligation to make an act of perfect contrition, including the intention of confessing as soon as possible (canon 916). A frequent reception of the Sacrament of Penance is encouraged for all.
For our fellow Christians
We welcome our fellow Christians to this celebration of the Eucharist as our brothers and sisters. We pray that our common baptism and the action of the Holy Spirit in this Eucharist will draw us closer to one another and begin to dispel the sad divisions which separate us. We pray that these will lessen and finally disappear, in keeping with Christ's prayer for us "that they may all be one" (Jn 17:21).
Because Catholics believe that the celebration of the Eucharist is a sign of the reality of the oneness of faith, life, and worship, members of those churches with whom we are not yet fully united are ordinarily not admitted to Holy Communion. Eucharistic sharing in exceptional circumstances by other Christians requires permission according to the directives of the diocesan bishop and the provisions of canon law (canon 844 §4). Members of the Orthodox Churches, the Assyrian Church of the East, and the Polish National Catholic Church are urged to respect the discipline of their own Churches. According to Roman Catholic discipline, the Code of Canon Law does not object to the reception of Communion by Christians of these Churches (canon 844 §3).
For those not receiving Holy Communion
All who are not receiving Holy Communion are encouraged to express in their hearts a prayerful desire for unity with the Lord Jesus and with one another.
We also welcome to this celebration those who do not share our faith in Jesus Christ. While we cannot admit them to Holy Communion, we ask them to offer their prayers for the peace and the unity of the human family.