"Sanctify them by Your truth; Your Word is Truth." (John 17:17) That is what Jesus prayed for His disciples—and for us, for a couple verse later Jesus says, "I do not pray for these (His disciples) alone, but also for those who will believe in Me through their word." That's us. Jesus prayed that we would be sanctified by God's Word of Truth. The road to sanctification is paved with the Word of God.
If you read through the whole prayer in John 17, you will find that it falls into three main paragraphs. You can tell that by the repeated words in each section. The first paragraph highlights the words glorify/glory and eternal life. The second paragraph repeats the words keep/kept and world. The third paragraph revolves around the words one and love/loved. Jesus' prayer for us to be sanctified by the truth of God's Word falls in the second paragraph.
So what? You might ask. As in realty, "Location. Location. Location." so also in studying the Bible: "Context. Context. Context." Context leads to proper interpretation, revealing clues to meaning and deeper understanding. This prayer for our sanctification is couched in Jesus' awareness of the influence of the world upon our lives. He doesn't pray that we'd been taken out of the world (vs. 15), which certainly seems like it would be the easier and faster route to sanctification and perfection in our minds; but He prays that we would be kept from the contamination of the world and from the evil one.
How will we be kept from the evil of the world? There's only one way: by the Word of Truth from God. This world is filled with lies. Every sin ever committed was and is based on a lie (Genesis 3:4-5). The enemy of our souls is the father of lies (John 8:44). We are so saturated in the lies of this world's system that we don't even recognize them for what they are.
Our own natural propensities and our natural ways of thinking are also based on lies for we are born in sin and sin-saturated. We don't often realize this. In fact, we think we're pretty good people most of the time. But once we begin to meditate on the Word of God we begin to see a different picture. The truth of God's Word upholds a very different standard than our own, a standard so elevated, so glorious, so beyond our reach, that we begin to realize we could never attain such a lofty benchmark on our own.
The Truth of God's Word leads us first of all to see our need for a Savior and a complete change of heart—a new heart—which is exactly what the Gospel offers us. Paul writes to Timothy (II Timothy 3:15) that the Holy Scriptures are "able to make you wise unto salvation." By meditating on the Truth of the Bible, we see the truth about ourselves, the falsehood of our own ways of thinking, the deceptive machinations of our culture, and our desperate need for the saving power of God.
We cannot think our way to Truth, but we can meditate our way to Truth by filling our minds with the Word of God and thinking deeply about it. Cogitate on the Word of God: reflect on it, consider it, contemplate it, mull over it, ponder it, muse upon it, ruminate on it, deliberate over it. It takes time, purposeful, intentional time. It also requires making the Word of God your deliberate focus. Through meditation on God's Word, you will be sanctified. "Sanctify them by your Truth; Your Word is Truth."