Charles H. Spurgeon on Psalms 119:66

Teach me good judgment and knowledge. Again he begs for teaching, as in verse 64, and again he uses God's mercy as an argument. Since God had dealt well with him, he is encouraged to pray for judgment to appreciate the Lord's goodness. Good judgment is the form of goodness which the godly man most needs and most desires, and it is one which the Lord is most ready to bestow. David felt that he had frequently failed in judgment in the matter of the Lord's dealings with him: from want of knowledge he had misjudged the chastening hand of the heavenly Father, and therefore he now asks to be better instructed, since he perceives the injustice which he had done to the Lord by his hasty conclusions. He means to say Lord, thou didst deal well with me when I thought thee hard and stern, be pleased to give me more wit, that I may not a second time think so ill of my Lord. A sight of our errors and a sense of our ignorance should make us teachable. We are not able to judge, for our knowledge is so sadly inaccurate and imperfect; if the Lord teaches us knowledge we shall attain to good judgment, but not otherwise. The Holy Ghost alone can fill us with light, and set the understanding upon a proper balance: let us ardently long for his teachings, since it is most desirable that we should be no longer mere children in knowledge and understanding.

For I have believed thy commandments. His heart was right, and therefore he hoped his head would be made right. He had faith, and therefore he hoped to receive wisdom. His mind had been settled in the conviction that the precepts of the word were from the Lord, and were therefore just, wise, kind, and profitable; he believed in holiness, and as that belief is no mean work of grace upon the soul, he looked for yet further operations of divine grace. He who believes the commands is the man to know and understand the doctrines and the promises. If in looking back upon our mistakes and ignorance we can yet see that we heartily loved the precepts of the divine will, we have good reason to hope that we are Christ's disciples, and that he will teach us and make us men of good judgment and sound knowledge. A man who has learned discernment by experience, and has thus become a man of sound judgment, is a valuable member of a church, and the means of much edification to others. Let all who would be greatly useful offer the prayer of this verse: "Teach me good judgment and knowledge."