1374 The mode of Christ’s presence under the Eucharistic species is unique. It raises the Eucharist above all the sacraments as “the perfection of the spiritual life and the end to which all the sacraments tend.” In the most blessed sacrament of the Eucharist “the body and blood, together with the soul and divinity, of our Lord Jesus Christ and, therefore, the whole Christ is truly, really, and substantially contained.” “This presence is called ‘real’ - by which is not intended to exclude the other types of presence as if they could not be ‘real’ too, but because it is presence in the fullest sense: that is to say, it is a substantial presence by which Christ, God and man, makes himself wholly and entirely present.”
1376 The Council of Trent summarizes the Catholic faith by declaring: “Because Christ our Redeemer said that it was truly his body that he was offering under the species of bread, it has always been the conviction of the Church of God, and this holy Council now declares again, that by the consecration of the bread and wine there takes place a change of the whole substance of the bread into the substance of the body of Christ our Lord and of the whole substance of the wine into the substance of his blood. This change the holy Catholic Church has fittingly and properly called transubstantiation.”
1380 It is highly fitting that Christ should have wanted to remain present to his Church in this unique way. Since Christ was about to take his departure from his own in his visible form, he wanted to give us his sacramental presence; since he was about to offer himself on the cross to save us, he wanted us to have the memorial of the love with which he loved us “to the end,” even to the giving of his life. In his Eucharistic presence he remains mysteriously in our midst as the one who loved us and gave himself up for us, and he remains under signs that express and communicate this love.
1381 “That in this sacrament are the true Body of Christ and his true Blood is something that ‘cannot be apprehended by the senses,’ says St. Thomas, ‘but only by faith, which relies on divine authority.’ For this reason, in a commentary on Luke 22:19 (‘This is my body which is given for you.’), St. Cyril says: ‘Do not doubt whether this is true, but rather receive the words of the Savior in faith, for since he is the truth, he cannot lie.’”
1390 Since Christ is sacramentally present under each of the species, communion under the species of bread alone makes it possible to receive all the fruit of Eucharistic grace. For pastoral reasons this manner of receiving communion has been legitimately established as the most common form in the Latin rite. But “the sign of communion is more complete when given under both kinds, since in that form the sign of the Eucharistic meal appears more clearly.” This is the usual form of receiving communion in the Eastern rites.