Day 188: Christ's Presence in the Eucharist

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... CCC 1375

In today's reading, the Catechism speaks of the Real Presence of Jesus Christ in the Eucharist. This isn't a symbolic or spiritual presence, but "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" (CCC 1374). This change the Church "has fittingly and properly called transubstantiation" (CCC 1376). The Catechism Compendium summarizes this teaching:

Transubstantiation means the change of the whole substance of bread into the substance of the Body of Christ and of the whole substance of wine into the substance of his Blood. This change is brought about in the eucharistic prayer through the efficacy of the word of Christ and by the action of the Holy Spirit. However, the outward characteristics of bread and wine, that is the “eucharistic species”, remain unaltered. (#283)

The Catechism Companion, Vol II comments on this:

From the beginning, the meaning of the Eucharist was recognized when many disciples walked away after Jesus spoke about his flesh and blood as true food and drink [Jn 6:52-66]... Church Fathers such as St. John Chrysostom and St. Ambrose emphasized the reality of the Eucharist as the true flesh and blood of Jesus Christ [CCC 1375]. The ability to "confect" the Eucharist is connected to the ministry and role of the priest, as expressed by St. John Chrysostom... Regardless of the size of the Communion Host we may receive, we receive the whole Christ. (p. 140) 

Finally, Dr. Brant Pitre has a good video on how the Eucharistic theology found in John 6 is misinterpreted:

Pope Benedict XVI celebrates a Mass.

Through the words of consecration prayed by the priest, bread and wine are converted into the Body and Blood of Christ (see CCC 1375). (p. 141)