"Before the greatnes of [the Eucharist] St. Augustine exclaims, ‘O sacrament of devotion! O sign of unity! O bond of charity!' The more painful the experience of the divisions in the Church which break the common participation in the table of the Lord, the more urgent are our prayers to the Lord that the time of complete unity among all who believe in him may return.
"The Eastern churches that are not in full communion with the Catholic Church celebrate the Eucharist with great love. ‘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, and so in the Eucharist, ‘given suitable circumstances and the approval of the Church authority, is not merely possible but is encouraged.
"Ecclesial communities derived from the Reformation and separated from the Catholic Church, ‘have not preserved the proper reality of the Eucharistic mystery in its fullness, especially because of the absence of the sacrament of Holy Orders.' It is for this reason that, for the Catholic Church, Eucharistic intercommunion with these communities is not possible. However these ecclesial communities, ‘when they commemorate the Lord's death and resurrection in the Holy Supper…profess that it signifies life in communion with Christ and await his coming in glory.'
When, in the Ordinary's judgment, a grave necessity arises, Catholic ministers may give the sacraments of Eucharist, Penance, and Anointing of the Sick to other Christians not in full communion with the Catholic Church, who ask for them of their own will, provided they give evidence of holding the Catholic faith regarding these sacraments and possess the required dispositions." (CCC 1398-1401)