There is a fourth important teaching in this stanza: not only 1) that we must begin to live for God when we are young; 2) that we must hide God's Word in our hearts, that is, that we must memorize it; and 3) that knowing the Bible is the only way to avoid sinning and live a pure life. The psalmist also teaches as a fourth truth that we cannot understand God's Word by ourselves, and therefore we need God himself for our teacher. Verse 12 notes this by coupling a line of praise with a line of petition. 

What can preserve us from ruin? What can empower us to say “No” to temptation? What can enable us to live a holy life in the midst of our most wicked surroundings? Only the Word of God, the Bible, which we must study and hide away in our hearts. Jesus told his disciples, “You are already clean because of the word I have spoken to you” (John 15:3). He also prayed to the Father on their behalf, saying, “Sanctify them by the truth,” noting that “your word is truth” (John 17:17). 

The first three verses of this section of Psalm 119 not only tell us that we should begin to live for God early in life—or, in case we are already older, at the earliest possible moment, that is, right now—they also tell us what we should do to actually live for God. It is to hide God's Word in our hearts, which means not just to read it but also to study it and even memorize it.

What is the condition of your heart, your home? Apart from the grace of God in your life it will always be occupied by such filthy evil spirits as lust, greed, pride and self-love. If you try to drive these demons out by yourself, they will only return in greater numbers and your latter state will be worse than at the first (Luke 11:24-26). God alone can cleanse the heart, and he does it through the agency of his Word, the Bible. 

This first stanza has moved from statements about the Bible as a source of blessing for all persons to a very personal resolution. If the psalm is to be helpful to us, it must become personal in our lives, too.