Rev 3:20, Gal 2:20
In the Old Testament, in the Song of Songs, Solomon asked his loved one to open the door. (Song 5:2-4)
Song 5:2 I sleep, but my heart waketh: it is the voice of my beloved that knocketh, saying, Open to me, my sister, my love, my dove, my undefiled: for my head is filled with dew, and my locks with the drops of the night. 3 I have put off my coat; how shall I put it on? I have washed my feet; how shall I defile them? 4 My beloved put in his hand by the hole of the door, and my bowels were moved for him.
Jesus, the Christ, knocks on the door of our hearts and wants to come into our hearts and live with us. (Revelation 3:20)
Revelation 3:20 Behold, I stand at the door, and knock: if any man hear my voice, and open the door, I will come in to him, and will sup with him, and he with me.
By believing in Jesus as the Christ, we died on the cross with Christ and rose again with Christ. It is no longer we who live in us, but Christ lives in us. (Ga 2:20)
Ga 2:20 I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me.