Catechism of the Catholic Church

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," ¹⁹⁸ 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