Because we believe that Christ’s living presence is contained in the Blessed Sacrament. The Blood is contained within the Body of a living person and the Body of a living person must contain the Blood. If the Body and Blood are separated from each other death occurs.
In the Eastern Catholic Churches, we have both forms given at the same time as the Body and Blood (bread and wine) is dropped on your tongue from a spoon.
In our tradition, you can’t have just one form, it has to be given in BOTH forms.
I know of one Catholic Church (Western) where Communion is only by intinction. This would violate the persons right to choose to receive in the hand. Until the Church retracts that right to choose, pastors should not take it upon themselves to force this.
It is up to the pastor to allow intinction or not. When communion under both species are offered in countries like Mexico and Brazil, the common method is Communion via intinction.
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