Reception of "the whole Christ"
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A blogger asked: "Why is that some churches offer the bread and wine and others just the bread?" I assume your question pertains to different Catholic parishes; I can't speak for the different parishes as to why they offer Holy Communion under one form or two. But, I can present the declaration of the United States Conference of Catholic Bishops which addresses Communion under both forms. Here are some excerpts (I've italicized the part that answers your question of why some parishes offer under both forms); to see the full text, please click on the title of this post.
"Christ is 'truly, really, and substantially contained' in Holy Communion. His presence is not momentary nor simply signified, but wholly and permanently real under each of the consecrated species of bread and wine...
The Council of Trent teaches that 'the true body and blood of our Lord, together with his soul and divinity, exist under the species of bread and wine. His body exists under the species of bread and his blood under the species of wine, according to the import of his words.'...
The Church also teaches and believes that 'immediately after the consecration the true body of our Lord and his true blood exist along with his soul and divinity under the form of bread and wine. The body is present under the form of bread and the blood under the form of wine, by virtue of the words [of Christ]. The same body, however, is under the form of wine and the blood under the form of bread, and the soul under either form, by virtue of the natural link and concomitance by which the parts of Christ the Lord, who has now risen from the dead and will die no more, are mutually united.'...
Since, however, by reason of the sign value, sharing in both eucharistic species reflects more fully the sacred realities that the Liturgy signifies, the Church in her wisdom has made provisions in recent years so that more frequent eucharistic participation from both the sacred host and the chalice of salvation might be made possible for the laity in the Latin Church...
The communicant makes this act of faith in the total presence of the Lord Jesus Christ whether in Communion under one form or in Communion under both kinds. It should never be construed, therefore, that Communion under the form of bread alone or Communion under the form of wine alone is somehow an incomplete act or that Christ is not fully present to the communicant... Thus, the Church has always taught the doctrine of concomitance, by which we know that under each species alone, the whole Christ is sacramentally present and we 'receive all the fruit of Eucharistic grace'...
At the same time an appreciation for reception of 'the whole Christ' through one species should not diminish in any way the fuller sign value of reception of Holy Communion under both kinds. For just as Christ offered his whole self, body and blood, as a sacrifice for our sins, so too is our reception of his Body and Blood under both kinds an especially fitting participation in his memorial of eternal life...
'Holy Communion has a more complete form as a sign when it is received under both kinds. For in this manner of reception a fuller sign of the Eucharistic banquet shines forth. Moreover there is a clearer expression of that will by which the new and everlasting covenant is ratified in the blood of the Lord and of the relationship of the Eucharistic banquet to the eschatological banquet in the Father's kingdom' (General Instruction to the Roman Missal, #281)."