Commentaries:
Sanctify means "set apart." It means, in this case, "make God the focus of one's thoughts, of one's approach to life, of the circumstance that one finds himself in."
Is this not what Peter forgot to do in Matthew 16? Satan's disinformation was the focus of Peter's response to Jesus—not God's thoughts. If God's thoughts or words had really been sanctified in Peter's heart at that time, he never would have said what he did. He would have said something like, "Yes, Jesus, I understand. That's what the scripture says." But instead, he disagreed with God. When one sanctifies God in his heart, then the Word of God becomes the focus, not the word of the spirit of this world.
John W. Ritenbaugh
Satan (Part 4)
Other Forerunner Commentary entries containing 1 Peter 3:15:
2 Timothy 3:16