Well, it’s ultimately a mystery. In lay terms, the outward appearances of the bread and wine remain. We believe it to ontologically be the Body and Blood of Christ by the power of the Spirit. Here are pertinent paragraphs in the Catechism:*
The presence of Christ by the power of his word and the Holy Spirit
1373 “Christ Jesus, who died, yes, who was raised from the dead, who is at the right hand of God, who indeed intercedes for us,” is present in many ways to his Church:197 in his word, in his Church’s prayer, "where two or three are gathered in my name,"199 in the poor, the sick, and the imprisoned,199 in the sacraments of which he is the author, in the sacrifice of the Mass, and in the person of the minister. But "he is present . . . most especially in the Eucharistic species."200
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."201 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."202 "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."203
1375 It is by the conversion of the bread and wine into Christ’s body and blood that Christ becomes present in this sacrament. The Church Fathers strongly affirmed the faith of the Church in the efficacy of the Word of Christ and of the action of the Holy Spirit to bring about this conversion. Thus St. John Chrysostom declares:
It is not man that causes the things offered to become the Body and Blood of Christ, but he who was crucified for us, Christ himself. The priest, in the role of Christ, pronounces these words, but their power and grace are God’s. This is my body, he says. This word transforms the things offered.204
And St. Ambrose says about this conversion:
Be convinced that this is not what nature has formed, but what the blessing has consecrated. The power of the blessing prevails over that of nature, because by the blessing nature itself is changed. . . . Could not Christ’s word, which can make from nothing what did not exist, change existing things into what they were not before? It is no less a feat to give things their original nature than to change their nature.205
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."206
1377 The Eucharistic presence of Christ begins at the moment of the consecration and endures as long as the Eucharistic species subsist. Christ is present whole and entire in each of the species and whole and entire in each of their parts, in such a way that the breaking of the bread does not divide Christ.207
Other worthwhile citations include the Council of Trent:“By the Consecration of the bread and of the wine a conversion is made 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 substrance of His blood: which conversion is by the Holy Catholic Church suitably and properly called transusbstantiation.”
…and from Trent’s 13th session, chapter 1:“substantially contained under the species of those sensible things.”*So you see how the physical properties of bread and wine remain to our perceptions. These physical properties are called the “species” or sometimes “accidents” of bread and wine. When we say the “substance” changes into Christ’s body, it does not refer to the physical properties of the bread and wine. We must understand the etymology of SUBstance, which means that which is beneath. The underlying reality of what a thing is, is called it’s “substance” in philosophical terms. It become’s Christ in reality, though we perceive bread and wine. Only by faith can we perceive Christ. (see also
1 Corinthians 11:29)
