Psalms 68:9

"When You went by in the desert, the earth was moved" [Psalm 68:8]. A desert were the nations, which knew not God: a desert they were, where by God Himself no law had been given, where no Prophet had dwelled, and foretold the Lord to come. "When," then, "You went by in the desert," when You were preached in the nations; "the earth was moved," to the faith earthly men were stirred up. But whence was it moved? "For the heavens dropped from the face of God." Perchance here some one calls to mind that time, when in the desert God was going over before His people, before the sons of Israel, by day in the pillar of cloud, by night in the brightness of fire; [Exodus 13:21] and determines that thus it is that "the heavens dropped from the face of God," for manna He rained upon His people: [Exodus 16:15] that the same thing also is that which follows, "Mount Sina from the face of the God of Israel," "with voluntary rain severing God to Your inheritance" [Psalm 68:9], namely, the God that on Mount Sina spoke to Moses, when He gave the Law, so that the manna is the voluntary rain, which God severed for His inheritance, that is, for His people; because them alone He so fed, not the other nations also: so that what next he says, "and it was weakened," is understood of the inheritance being itself weakened; for they murmuring, fastidiously loathed the manna, longing for victuals of flesh, and those things on which they had been accustomed to live in Egypt. [Numbers 11:5-6] ... Lastly, all those men in the desert were stricken down, nor were any of them except two found worthy to go into the land of promise. [Numbers 14:23-24] Although even if in the sons of them that inheritance be said to have been perfected, we ought more readily to hold to a spiritual sense. […]

You will set apart a voluntary rain, O God, for your inheritance. He speaks of the law, which was given through Moses. And it was weakened, but you have perfected it. The law was weakened, because no one was able to fulfill it except the Lord, who said: "I have not come to abolish the law, but to fulfill it." And according to the apostle: "For what was impossible for the law, in that it was weakened, God sent his own Son," and the rest.

And first we have had to be persuaded how much God loved us, lest from despair we should not dare to look up to Him. And we needed to be shown also what manner of men we are whom He loved, lest being proud, as if of our own merits, we should recede the more from Him, and fail the more in our own strength. And hence He so dealt with us, that we might the rather profit by His strength, and that so in the weakness of humility the virtue of charity might be perfected. And this is intimated in the Psalm, where it is said, "Thou, O God, didst send a spontaneous rain, whereby Thou didst make Thine inheritance perfect, when it was weary." For by "spontaneous rain" nothing else is meant than grace, not rendered to merit, but given freely, whence also it is called grace; for He gave it, not because we were worthy, but because He willed. And knowing this, we shall not trust in ourselves; and this is to be made "weak." But He Himself makes us perfect, who says also to the Apostle Paul, "My grace is sufficient for thee, for my strength is made perfect in weakness." Man, then, was to be persuaded how much God loved us, and what manner of men we were whom He loved; the former, lest we should despair; the latter, lest we should be proud.

This looks further to the spiritual provision made for God's Israel; the Spirit of grace and the gospel of grace are the plentiful rain with which God confirms his inheritance, and from which their fruit is found, Isa. xlv. Christ himself is this rain, Ps. lxxii.

Thou, O God, didst send a plentiful rain,....