13 Since then the aforesaid republic is both the subject on which this book was written, and it is now proposed for us to speak thereof, let us give careful heed to Matthew, discoursing plainly concerning this: for what he says is not his own, but all Christ's, who has made the laws of this city. Let us give heed, I say, that we may be capable of enrolment therein, and of shining forth among those that have already become citizens thereof, and are awaiting those incorruptible crowns. To many, however, this discourse seems to be easy, while the prophetic writings are difficult. But this again is the view of men who know not the depth of the thoughts laid up therein. Wherefore I entreat you to follow us with much diligence, so as to enter into the very ocean of the things written, with Christ for our guide at this our entering in.
But in order that the word may be the more easy to learn, we pray and entreat you, as we have done also with respect to the other Scriptures, to take up beforehand that portion of the Scripture which we may be going to explain, that your reading may prepare the way for your understanding (as also was the case with the eunuch), and so may greatly facilitate our task.
Source: Homilies on the Gospel of St. Matthew (New Advent)