Ruin in abundance, O God, You shed abroad; You restored Your inheritance as it languished.
This is to signify that the preaching of the New Testament, since Christ's true exodus from this world, will far excel that of the old Law. For while it rained sparsely there, it is to rain in abundance here; while it descended only on one spot there, here it will be diffused over all parts of the world; while it affected only one nation there, Israel, here a general shower will be dispersed over all, Gentile and Jew. The Gospel will not be confined to one country and one nation as the proclamation of the Law was. Those drops were released by the heavens, too, by the angels through Moses, in God's stead, as St. Paul declares in Galatians 4. But this rain Thou, O God, wilt distribute Thyself. "Free rain" [cf. Psalm 67:10 Vulgate] could also be interpreted to mean that the teaching of the Gospel is free and that it engenders free hearts, which are no longer bound to externals but live freely by faith. That is the Christian liberty of which Psalm 110:3 speaks: "Thy people will be spontaneous."
From Commentary on Psalm 68 (Luther's Works 13:10)