2 In the book of the prophet it is written: “Take to yourself the rod of an almond tree.” We ought to consider why the Lord said this to the prophet, for it was not written without a purpose, since in the Pentateuch too we read that the almond rod of Aaron the priest, after being long laid up, blossomed. For the Lord seems to signify by the rod that the prophetic or priestly authority ought to be straightforward, and to advise not so much what is pleasant as what is expedient.
Source: Letters (New Advent)