Concerning the Orthodox churches, who primarily disagree with Catholics over the authority of the Pope, Vatican II’s “Decree on Ecumenism” (“Unitatis Redintegratio,” 1964) stated, “These Churches, although separated from us, yet possess true sacraments, above all — by apostolic succession — the priesthood and the Eucharist, whereby they are still joined to us in closest intimacy.” A certain communion in sacris including the Holy Eucharist, “given suitable circumstances and the approval of Church authority, is not merely possible but is encouraged”
A Catholic must be in a state of grace to receive Holy Communion, and anyone aware of being in a state of mortal sin must first receive absolution in the Sacrament of Penance (Catechism, No. 1415). Therefore, a non-practicing Catholic who has negligently not attended Mass or who has abandoned the teachings of the Church is not in a state of grace and cannot receive Holy Communion. A non-practicing Catholic who receives Holy Communion commits the sin of sacrilege — the abuse of a sacrament — and causes scandal among the faithful. St. Paul reminded the Corinthians: “Every time, then, you eat this bread and drink this cup, you proclaim the death of the Lord until He comes! This means that whoever eats the bread or drinks the cup of the Lord unworthily sins against the body and blood of the Lord. A man should examine himself first; only then should he eat of the bread and drink of the cup” (1 Cor 11:26-28).
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