1 Peter 3:15 but sanctify Christ as Lord in your hearts, always being ready to make a defense to everyone who asks you to give an account for the hope that is in you, yet with gentleness and reverence; (NASB: Lockman)
In the Gospels "the Christ" is not a personal name but an official designation for the expected Messiah (Mt 2:4, Lk 3:15). As by faith the human Jesus was recognized and accepted as the personal Messiah, the definite article ("the") was dropped and the designation "Christ" came to be used as a personal name. The name "Christ" speaks of His Messianic dignity and emphasizes that He is the fulfillment of the Old Testament promises concerning the coming Messiah.
Lord (kurios from kúros = might, power in turn from kuróo = give authority, confirm) describes One who has absolute ownership. Kurios describes the One who has sovereign power and authority. Kurios also conveys the idea of master. Thus, the second Person of the Trinity was to be Lord and Master of their lives. He was to be their resource and defender when persecution came.
Heart (2588) (kardia) does not refer to the physical organ but is always used figuratively in Scripture to refer to the seat and center of human life. The heart is the center of the personality, and it controls the intellect, emotions, and will.
MacArthur commenting on kardia writes that..."While we often relate heart to the emotions (e.g., "He has a broken heart"), the Bible relates it primarily to the intellect (e.g., "Out of the heart come evil thoughts, murders, adulteries, fornications, thefts, false witness, slanders," Matt 15:19). That’s why you must "watch over your heart with all diligence" (Pr 4:23).
Now, let me sum up my exposition. The fruits of the sanctified life are to be found in the tender graces and in commanding virtues, in compassion, sensitive and humbleminded (1Pe 3:8), and in moral and spiritual enthusiasm which is perfectly devoid of fear. Now, do you not think that where these soft compassions flow and these sterner virtues dwell—river and rock—a man will be able to "give answer to every man that asketh a reason concerning the hope that is in him"? [1Peter 3:15]