In Chapter 62 of Book IV of Summa Contra Gentiles, St. Thomas outlined reasonable difficulties in believing in the Real Presence of Christ in the Blessed Sacrament of the altar. The last one involved our custom at Mass of breaking the Host.
…It even seems absurd to say that the subject of the breaking of the bread at the Mass is the Body of Christ…
Since Christ’s Body, risen from the dead, and subject to no injury or corruption at this point, cannot be broken.
Next, St. Thomas comments on the saying of Christ that supported all the objections to the Real Presence, Namely:
The words that I have spoken to you are spirit and life.
Finally, to conclude the section on the Holy Eucharist, St. Thomas considers the use of leavened vs. unleavened bread.
The next section of Book IV considers the Sacrament of Penance. St. Thomas begins by considering whether an initiated Christian can sin.