Catena Aurea on Matthew 7:6: Augustine of Hippo

Let us see now what is the holy thing, what are the dogs, what the pearls, what the swine? The holy thing is all that it were impiety to corrupt; a sin which may be committed by the will, though the thing itself be undone. The pearls are all spiritual things that are to be highly esteemed. Thus though one and the same thing may be called both the holy thing and a pearl, yet it is called holy because it is not to be corrupted; and called a pearl because it is not be contemned.