At a recent Mass, the priest invited all Christians to come forward to receive Communion. That didn’t seem right to me. What is the rule is for non-Catholics to receive the Eucharist?
The Code of Canon Law allows for some non-Catholics to receive the Eucharist:
Catholic ministers administer the sacraments of penance, Eucharist, and anointing of the sick licitly to members of Eastern Churches which do not have full communion with the Catholic Church if they seek such on their own accord and are properly disposed. This is also valid for members of other Churches which in the judgment of the Apostolic See are in the same condition in regard to the sacraments as these Eastern Churches. (CIC 844 §3)
Other non-Catholic Christians may receive the Eucharist only under specific circumstances:
If the danger of death is present or if, in the judgment of the diocesan bishop or conference of bishops, some other grave necessity urges it, Catholic ministers administer these same sacraments licitly also to other Christians not having full communion with the Catholic Church, who cannot approach a minister of their own community and who seek such on their own accord, provided that they manifest Catholic faith in respect to these sacraments and are properly disposed. (CIC 844 §4)