15 The sacramental re-presentation of Christ's sacrifice, crowned by the resurrection, in the Mass involves a most special presence which – in the words of Paul VI – “is called 'real' not as a way of excluding all other types of presence as if they were 'not real', but because it is a presence in the fullest sense: a substantial presence whereby Christ, the God-Man, is wholly and entirely present”. This sets forth once more the perennially valid teaching of the Council of Trent: “the consecration of the bread and wine effects the 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. And the holy Catholic Church has fittingly and properly called this change transubstantiation”. Truly the Eucharist is a mysterium fidei , a mystery which surpasses our understanding and can only be received in faith, as is often brought out in the catechesis of the Church Fathers regarding this divine sacrament: “Do not see – Saint Cyril of Jerusalem exhorts – in the bread and wine merely natural elements, because the Lord has expressly said that they are his body and his blood: faith assures you of this, though your senses suggest otherwise”. Adoro te devote, latens Deitas , we shall continue to sing with the Angelic Doctor. Before this mystery of love, human reason fully experiences its limitations. One understands how, down the centuries, this truth has stimulated theology to strive to understand it ever more deeply. These are praiseworthy efforts, which are all the more helpful and insightful to the extent that they are able to join critical thinking to the “living faith” of the Church, as grasped especially by the Magisterium's “sure charism of truth” and the “intimate sense of spiritual realities” which is attained above all by the saints. There remains the boundary indicated by Paul VI: “Every theological explanation which seeks some understanding of this mystery, in order to be in accord with Catholic faith, must firmly maintain that in objective reality, independently of our mind, the bread and wine have ceased to exist after the consecration, so that the adorable body and blood of the Lord Jesus from that moment on are really before us under the sacramental species of bread and wine”.
Source: Ecclesia de Eucharistia (Vatican.va)