From Commentary on Psalm 118, 12-14 (386-390)
Let your door stand open to receive Christ, unlock your soul to him, offer him a welcome in your mind, and then you will see the riches of simplicity, the treasurers of peace, and the joy of grace. Throw wide the gate of your heart, stand before the sun of the everlasting light that shines on every one. This true light shines on all, but if any close their windows they will deprive themselves 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. 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 son of holiness never sets, because wisdom cannot give place to evil.
Blessed then is the person 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 make his departure. If you were to go to sleep, if your heart were not wide awake, he would not knock and go away; but if your heart is watchful, he knocks and asks you to open the door to him.
Our soul has a door; it has gates. “Lift up your heads, O gates, and be lifted up, eternal gates, and the King of glory will enter.” If you open the gates of your faith, the King of glory will enter your house in the triumphal procession in honor of his passion. Holiness, too, has its gates. We read in Scripture what the Lord Jesus said through his prophet: “Open me the gates of holiness.”
It is the soul that has its door, its gates. Christ comes to this door and knocks; he knocks at the gates. Open to him; he wants to enter, to find his bride waiting and watching.
St. Ambrose (ca. 334-397) became Archbishop of Milan at a time of bitter strife about Christological doctrine, and upheld orthodox teaching in a his widely circulated sermons and treatises. He is credited with introducing hymns to the Western Church, some composed by himself, and his greatest disciple was the even more influential St. Augustine. His commentary on Psalm 118 (119), based on a series of earlier sermons, was published near the end of his life. His feast day is December 7.