I Peter 3:15, But sanctify the Lord God in your hearts: and be ready always to give an answer to every man that asketh you a reason of the hope that is in you with meekness and fear:
Sanctifying the Lord in your heart does not make Jesus holy, but it is recognizing that He is holy. It is to stand in awe of HIs holiness, have confidence in His power, love in our hearts for Him, and to obey His commands. Being ready to give an answer and witness for Jesus requires submitting the Lordship of Christ in your life and knowing our Holy God.
This passage is a reference from Isaiah 8:13. In Isaiah’s time, the people were not trusting the Lord and some went to peeping and muttering wizards and soothsayers for deeper understanding and guidance. The conventional wisdom was that people should get their direction from God by speaking to the dead. They lived in very spiritual times, but being a spiritual person is only good when you follow the right spirit. Israel preferred the unsure words of devil worshippers above the Word of God. Isaiah answered this error with the call to “sanctify the LORD of hosts himself; and let him be your fear, and let him be your dread” (Isa. 8:13) and when they come with their wisdom or spirituality, you go “to the law and the testimony: if they speak not according to this word, it is because there is no light in them” (Isa. 8:20). Trust in the Lord and heed to His Word. In the context of 1 Peter 3, God’s people are following the good, even if it leads to persecution.
They stand against the wisdom of the age and follow their Lord, not fearing the threat, nor being afraid of the trouble that comes when we do. Who can really harm you if you follow the Lord? How then do we sanctify the Lord in our hearts? To the law and testimony. We hold to and follow the Word of God and the God of the Word. Regardless of popular opinion, or the sincerity of the other message, Jesus is Lord and His word stands. Sanctifying God in your heart is to take God’s word as the foundation of your life, no matter the circumstances. If we are defending the faith against someone who is opposed to Christianity, or witnessing to someone who doesn’t believe, we must stand on the firm and solid truth that Jesus is Lord, Jesus is Truth. We won’t sway to popular ideas or conventional wisdom. Our hope does not come from our feelings and intuitions. Our confidence is not on the wisdom from below, but from above. Jesus is Lord, God is sovereign, His Word is powerful, and so we witness for Him believing those truths. When we see the holiness of our God, we will fear him. When you fear God, you have nothing else to fear.