The Holy Communion is the third of the Sacraments of Initiation. Even though we are required to receive Communion at least once per year (which is our Easter Duty), and the Church urges us to receive Communion frequently (even daily, if possible), it is called a sacrament of initiation because, like Baptism and Confirmation, it brings us into the fullness of our life in Christ.
Normally, only Catholics in a state of grace can receive the Sacrament of Holy Communion. Under certain circumstances, however, other Christians whose understanding of the Eucharist (and the Catholic sacraments generally) is the same as that of the Catholic Church can receive Communion, even though they are not in full communion with the Catholic Church.
The U.S. Conference of Catholic Bishops in their Guidelines for the Reception of Communion, notes that: “Eucharistic sharing in exceptional circumstances by other Christians requires permission according to the directives of the diocesan bishop and the provisions of canon law.”
In those circumstances, 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.
Under no circumstances are non-Christians allowed to receive Communion, but Christians beyond those mentioned above (e.g., Protestants) can, under canon law Canon 844, Section 4, receive Communion in very rare circumstances:
If the danger of death is present or other grave necessity, in the judgment of the diocesan bishop or the conference of bishops, Catholic ministers may administer these sacraments to other Christians who do not have full Communion with the Catholic Church, who cannot approach a minister of their own community and on their own ask for it, provided they manifest Catholic faith in these sacraments and are properly disposed.