Charles H. Spurgeon on Psalms 94:12

And teachest. Teaching implies both a schoolmaster, a teacher, instructing and lessons taught. In this teaching both these points are here noted out. And for the first, namely, the schoolmaster, it is twofold: 1. The outward affliction and chastisement, "Whom you chastise, teach, "that is, whom by chastising you teach. 2. God himself, who is the chief and principal head schoolmaster, the other being but an inferior and subordinate one: "Whom thou teachest." And for the second point, the lessons taught, they are included generally in those words, "in thy law." To begin then with the schoolmasters, and first with the first.

The first schoolmaster is affliction. A sharp and severe and swingeing schoolmaster indeed, and so much the fitter for such stout and stubborn scholars as we are; who because we will not be overcome by fair means, must needs therefore be dealt withal by foul. For God doth not willingly afflict us, but being necessarily thereunto enforced, by that strength of corruption in us, which otherwise will not be subdued. So physicians and surgeons are constrained to come to cutting, lancing, and burning, when milder remedies will not prevail. Let us therefore hereby take notice of the hardness of our hearts, the fallow ground whereof cannot be broken up but by this sharp plough of affliction. See what dullards and blockheads we are, how slow to understand spiritual things, not able to conceive of them by the instruction of words, unless they be even beaten and driven into our brains by blows. So thick and brawny is that foreskin which is drawn over our uncircumcised ears and hearts, that no doctrine can enter, unless it be pegged, and hammered, and knocked into us by the fists of this sour and crabbed schoolmaster.

The second schoolmaster is God himself. Afflictions of themselves, though severe schoolmasters, yet can do us no good, unless God come by his Spirit, and teach our hearts inwardly. Let us therefore pray that as in the ministry of God's word, so also of his works and judgments, we may be all taught of God. For it is his Spirit that quickens and animates the outward means, which otherwise are a dead letter. And this is the reason that many men have rather grown worse by their afflictions, than anything better; because God's Spirit hath not gone with the affliction, to put life and spirit into it, as Moses observed in the Israelites, De 29:24. David Dyke.