Why can't non-Catholics receive Communion?
Why does the Church practice closed Communion? If unity is caused by sharing in the Lord's Table, then why not let everybody receive, whether they are Catholic or not?
Because unity is not caused by sharing in the Lord’s Table any more than a marriage is caused by intercourse. For those who are out of full communion with the Catholic Church, the spiritual unity must happen first (by entering into full unity with the Church through the rites of initiation), just as those who are not married must be married before they can enter into bodily unity. In both cases, the physical unity naturally flows from the spiritual unity. Eucharist strengthens the unity of the Body of Christ, just as the marriage act strengthens the sacrament of matrimony. But we must do first things first!