At that time, there were many who welcomed Jesus in the flesh and ate and drank with him; but because they did not welcome him in spirit, they remained in their misery. The blessed Mary herself, whose glorious assumption we are celebrating, was beyond doubt blessed because she welcomed the Son of God in body; but she was more blessed because she had welcomed him in spirit.
Let us therefore prepare a spiritual castle and the Lord will come to us. I dare say that if the blessed Mary had not prepared this castle within herself, the Lord Jesus would not have entered her womb or spirit. Let us then make ready such a castle. Three things make a castle strong: a moat, a wall and a tower. The wall and the moat protect one another, since–if the moat were not in the way, people would be able to devise a way of approaching the wall to undermine it. And if the wall did not rise above the moat, they could approach the moat to fill it in. The tower, on the other hand, protects the whole because it is higher than the other parts.
Let us now enter our soul and let us see how this ought to be actualized spiritually. What is the moat but a deep hollow? Therefore, let us hollow out our heart so that it may be very low ground. Let us take away the earth which is inside and heave it up for that is how a moat is made. The earth we should take and mound up is our earthly fragility. Let this not lie hidden within but let it always be before our eyes, so that in our heart there may be a moat–that is a low-lying, deep ground. This moat is humility.
In this way, let us begin to build this castle. For unless this moat–true humility–is first established in our heart, we would only build something that will fall in ruins over our heads. How well the blessed Mary had made this moat for herself! truly, she was more mindful of her own fragility than of all her dignity and holiness. She knew full well that the fact that she was fragile came from herself, that the fact that she was holy, that she was the Mother of God, that she was the Lady of the angels and the temple of the Holy Spirit, came only by God’s grace. Therefore what she was of herself she humbly confessed, saying: Behold the handmaid of the Lord; be it done to me according to your word. And again: He looked upon the humility of his handmaid.
from Aelred of Rievaulx: The Liturgical Sermons, Serrmon 19 for the Assumption of St. Mary, CF 58, pp. 264-265