Communion means being in union with God and with one another in our beliefs. If there isn’t unity in faith, morals, and sacramental life, then common reception of the Blessed Sacrament is prohibited.
The United States Catholic Conference issued a set of guidelines for the reception of Holy Communion which has to be printed in every missalette and Mass program. The guidelines basically point out what the Code of Canon Law directs Catholics, Protestants, and non-Christians to do at Mass.
Canon 916 reminds us that Catholics are encouraged to receive Holy Communion provided that they are properly disposed to reception. What is proper disposition? A Catholic should not be conscious of any mortal sins and should have fasted for one hour before Communion. If they are conscious of grave sin, then the Catholic must go to Confession before reception. There is one exception which the canon points out: “unless there is grave reason and there is no opportunity to confess; in this case the person is to remember the obligation to make an act of perfect contrition, which includes the resolve to go to confession as soon as possible.”
Baptized non-Catholics are certainly welcome to attend Holy Mass. Baptism makes us members of the Church, and so there is the hope of baptized non-Catholics one day being in full communion with the Church. Since unity of belief does not yet exist, though, reception of Holy Communion is not possible. Canon Law provides a few exceptions. Canon 844, Paragraph 4 states, “If there is a danger of death or if, in the judgment of the diocesan bishop or the Bishop’s Conference, there is some other grave and pressing need, Catholic ministers may lawfully administer those same sacraments to other Christians not in full communion with the Catholic Church, who cannot approach a minister of their own community and who spontaneously ask for them, provided that they demonstrate the Catholic faith in respect of these sacraments and are properly disposed.”
Members of the Orthodox Churches, the Assyrian Church of the East, and the Polish National Catholic Church are directed by Canon 844, paragraph 3, “Catholic ministers may lawfully administer the Sacraments of Penance, the Eucharist, and Anointing of the Sick to members of the Eastern Churches not in full communion with the Catholic Church if they spontaneously ask for them and are properly disposed.”
Finally, non-Christians are certainly welcomed to Mass but are not allowed to receive Holy Communion under any circumstances, since they do not share the same belief in Jesus Christ.
Catholics who cannot receive Holy Communion because of mortal sin should make a Spiritual Communion and go to the sacrament of Penance as soon as possible.