From an exposition of psalm 118 by Saint Ambrose, bishop
God’s temple is holy; you are his temple
My Father and I will come and make our home with him. Let your door stand open to receive him, unlock your soul to him, offer him a welcome in your mind, and then you will see the riches of simplicity, the treasures of peace, the joy of grace. Throw wide the gate of your heart, stand before the sun of the everlasting light that shines on every man. This true light shines on all, but if anyone closes his window he will deprive himself of eternal light. If you shut the door of your mind, you shut out Christ. Though he can enter, he does not want to force his way in rudely, or compel us to admit him against our will.
Born of a virgin, he came forth from the womb as the light of the whole world in order to shine on all men. His light is received by those who long for the splendor of perpetual light that night can never destroy. The sun of our daily experience is succeeded by the darkness of night, but the sun of holiness never sets, because wisdom cannot give place to evil.
Blessed then is the man at whose door Christ stands and knocks. Our door is faith; if it is strong enough, the whole house is safe. This is the door by which Christ enters. So the Church says in the Song of Songs: The voice of my brother is at the door. Hear his knock, listen to him asking to enter: Open to me, my sister, my betrothed, my dove, my perfect one, for my head is covered with dew, and my hair with the moisture of the night.
When does God the Word most often knock at your door? — When his head is covered with the dew of night. He visits in love those in trouble and temptation, to save them from being overwhelmed by their trials. His head is covered with dew or moisture when those who are his body are in distress. That is the time when you must keep watch so that when the bridegroom comes he may not find himself shut out, and take his departure. If you were to sleep, if your heart were not wide awake, he would not knock but go away; but if your heart is watchful, he knocks and asks you to open the door to him.