Deuteronomy 32:2

2My doctrine shall drop as the rain. Some, as I think improperly, here resolve the future tense into the optative mood, (248) for in this splendid eulogium he rather celebrates, in order to commend his doctrine, the fruitfulness (249) which is actually imparted to it by the Holy Spirit, than asks for it to be given to him; and my readers must at once perceive that such a request would have been by no means seasonable. He therefore compares his speech to rain or dew, as if he had said that, if only the people were like the soil in a state of softness and preparation, he would deliver doctrine to them which would irrigate them unto abundant fruitfulness.

Although this expression refers especially, and κατ ᾿ ἐξοχὴν to the Song, still its force and propriety extends to all divine teaching; for God never speaks except to render men fruitful in good works, just as, by instilling succulency and vigor into the earth by means of rain, He makes it fertile for the production of fruit. But, like the rocks and stones, which imbibe no moisture from the most abundant rains, so many are hindered by their own perversity from being fertilized by spiritual irrigation. Wherefore Moses indirectly throws the blame upon the Israelites, if the doctrine of this Song should drop upon them in vain.