"By the consecration the transubstantiation of the bread and wine into the Body and Blood of Christ is brought about. Under the consecrated species of bread and wine Christ himself, living and glorious, is present in a true, real, and substantial manner: his Body and his Blood, with his soul and his divinity (cf. Council [...]
"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 [...]
"Since Christ our Redeemer declared that to be truly His own body which He offered under the form of bread, it has, therefore, always been a firm belief in the Church of God, and this holy council now declares it anew, that by the consecration of the bread and wine a change is brought about [...]
"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. [...]
"In the Mass, the faithful are already present on the altar under the appearance of bread and wine. At the moment of the Consecration of the Mass, when the priest as Christ pronounces the words 'This is My Body' and 'This is My Blood,' the substance of the bread becomes the substance of the body [...]
"In this sacrament the consecration of the matter consists in the miraculous change of the substance, which can only be done by God; hence the minister in performing this sacrament has no other act save the pronouncing of the words." Source: St. Thomas Aquinas, Summa Theologica
"The accidents continue in this sacrament without a subject. This can be done by Divine power: for since an effect depends more upon the first cause than on the second, God Who is the first cause both of substance and accident, can by His unlimited power preserve an accident in existence when the substance is [...]
"He once in Cana of Galilee, turned the water into wine, akin to blood, and is it incredible that He should have turned wine into blood?" Source: St. Cyril of Jerusalem, Catechetical Lectures
"Form cannot be changed into form, nor matter into matter by the power of any finite agent. Such a change, however, can be made by the power of an infinite agent, which has control over all being, because the nature of being is common to both forms and to both matters; and whatever there is [...]
"Consequently, no natural or created agent can act except by changing the form in something; and on this account every change made according to nature's laws is a formal change. But God is infinite act… hence His action extends to the whole nature of being. Therefore He can work not only formal conversion, so that [...]