When we say "heart," we mean deep down inside us - where we really feel and believe. So when someone says, "Jesus lives in my heart," the person means that he has asked Jesus to be his Saviour - to forgive and take care of him - and that Jesus is in charge of his life. When someone asks Jesus to take over, God really does come inside - the Holy Spirit comes and lives inside that person. And the Holy Spirit can be in all of the people who love God at the same time. Jesus wants to be very close to you, too, like a good friend. Through his Holy Spirit, he wants to "live in your heart."
He has kept this secret for centuries and generations past. But now he is pleased to tell it to the brothers and sisters. The riches and glory of his plan are for you Gentiles, too. This is the secret: Christ in your hearts is your only hope of glory. (Colossians 1:26-27)
John 16:7-8; Acts 1:8; 1 Thessalonians 4:8
Note to Parents
Children are often told that Jesus comes to live inside their hearts when they believe in him. Young children take this literally and think that a miniaturized Jesus actually lives in their chest. "Jesus lives in my heart" is a shorthand way of calling Jesus Saviour and Lord. "Jesus goes with you everywhere" may be a better way of phrasing it for young children.