John Calvin Commentary Psalms 119:9

"Wherewith shall a young man cleanse his way? By taking heed [thereto] according to thy word." — Psalms 119:9 (ASV)

Wherewith shall a young man cleanse his way? In this place, he repeats, in different words, the same truth which he previously stated: that however much people may pride themselves on their own works, there is nothing pure in their life until they have made a complete surrender of themselves to the word of the Lord.

To more effectively motivate them to this, he presents, in a special way, the example of children or young people. In mentioning these, he by no means gives unrestricted freedom to those who have reached mature years, or who are elderly, as if they were capable of managing their own lives, and as if their own wisdom served as their law. Instead, because youth places people at a crossroads and makes it essential for them to choose the path in life they intend to follow, he declares that when a person begins to order their life, no advice will be of any benefit unless they adopt the law of God as their rule and guide.

In this way, the prophet encourages people to an early and timely ordering of their conduct, and not to postpone it any longer, according to the words of Solomon: “Remember thy Creator in thy youth, ere the days of trouble come, and the years which shall be grief unto thee” (Ecclesiastes 12:1). Those who continually postpone become hardened in their sinful practices and reach mature years when it is too late to attempt reform.

There is another reason: fleshly desires are very powerful in youth, requiring a double restraint. The more young people are inclined to excess, the greater the need to restrain their immoral desires. Therefore, the prophet, with good reason, particularly urges them to pay attention to observing the law.

We may reason from the greater to the lesser: for if the law of God has the power to restrain the impulsiveness of youth, so as to keep pure and upright all who take it as their guide, then, surely, when they reach maturity and their unruly desires have significantly lessened, it will prove to be the best remedy for correcting their sins.

Therefore, the reason so much evil prevails in the world is that people indulge in their own impurity and are inclined to follow their own desires more than heavenly instruction. The only sure protection is to order our lives according to God’s word. Some, wise in their own estimation, throw themselves into Satan’s snares; others, out of apathy and sluggishness, live a vile and wicked life.